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The expert
(Pic: Shutterstock)

28 Feb 2024 / justice Print

The expert

Flawed expert evidence can lead to a court, acting in good faith, reaching an unsound decision. Brian McMullin examines the fundamental importance of expert evidence to the rule of law – and the expert’s primary duty

Mr Justice Peart, retired judge of the Court of Appeal, observed of expert witnesses at a webinar in 2021 that: “Very often, the names that appear on a plaintiff’s side never appear on a defendant’s side, and vice versa.”

In addition, he pointed out that the primary duty of experts “is to the court and not to the parties that are paying them”.

In consideration of the best use of experts, we should firstly start with the end result, and the purpose of experts participating in court proceedings and the hearing.

Firstly, this participation is a fundamental part of the rule of law. The rule of law can fall down for various reasons – for example, objective bias, unnecessary delay in getting justice, a difficulty in enforcing a decision – but, fundamentally relevant to this discussion, flawed evidence may have been adduced.

Flawed evidence

Flawed evidence can lead to miscarriages of justice and, in turn, to a lack of confidence in justice and a degradation of the rule of law.

That is why expert evidence is absolutely fundamental to the rule of law, as flawed expert evidence can lead to a court acting in good faith reaching an unsound decision.

The second most important point already alluded to is that the expert’s primary duty is to the court, and not to their client who will be paying his or her fees. It is useful to consider some of the relevant case law, starting with the UK House of Lords decision in Whitehouse and Jordan in 1981.

In that case, Lord Wilberforce points out that, while some degree of consultation between experts and legal advisors is entirely proper and necessary, the expert evidence presented to the court should be, and should be seen to be, the independent product of the expert uninfluenced as to form or content by the exigencies of litigation.

This was presumably his way of saying that it should not be influenced by the narrow self-interests of the client.

Lady Justice Carr in the Secretariat case later put it very succinctly in that judgment when she said that “an expert who complies fully with his duty of independence and objectivity to the court or arbitral tribunal is an expert who provides his client with the best possible service”.

Key principles

Mr Justice Cresswell in the Ikarian Reefer case in 1993 identified key principles and responsibilities of expert witnesses.

He pointed out that the list was not exhaustive and, after reaffirming the comments of Lord Wilberforce in Whitehouse v Jordan as regards independence, he went on to set out his principles as follows:

1) Such witnesses should provide independent assistance to the court by way of objective, unbiased opinion in relation to matters within their expertise, and should never act as advocates.

2) Such witnesses should state the facts or assumptions upon which their opinion is based, and consider material facts that could detract from their concluded opinion.

3) Expert witnesses should make it clear when a particular question or issue is outside their expertise.

4) If such witnesses consider that insufficient data is available, they should say so, and indicate that the opinion is provisional only.

5) If the witness is not sure that their report contains the truth, the whole truth and nothing but the truth, without some qualification, they should state that qualification in their report. If an expert witness changes his views on a material matter, such change of views should be communicated (through the lawyers) to the other side without delay and, when appropriate, to the court.

6) Where expert evidence refers to photographs, plans, calculations, analyses, measurements, survey reports, or other similar documents, these must be provided to the opposite party at the same time as the exchange of reports.

These duties and responsibilities have been considered with approval in our courts, as noted most recently by Noonan J in Duffy v McGee (2022), when he commented: “The overriding duty of the expert is owed to the court and includes the duty to provide an objective opinion. Objectivity, by definition, requires that one has regard to both sides of the case. A central component of the duty of the expert is to ascertain all relevant facts, whether they support the client’s case or not.”

He further observed that: “It is unfortunately commonplace for experts to succumb to the natural tendency to put the interests of their own clients first, unconsciously or otherwise.”

Danger of ‘hired guns’

In essence, the expert must be independent, provide an objective and unbiased opinion, and his or her opinion should be based on relevant, accurate, and complete information. The expert should not, in any sense, be a ‘hired gun’ perceived by the litigant to be favourable to their case.

Collins J, in the Duffy case, observed that “arguably, the most significant concern about expert evidence relates to issues of objectivity, impartiality and independence”.

Concerns of that kind prompted Cresswell J to formulate a detailed statement of the duties and responsibilities of expert witnesses in the Ikarian Reefer, and “variants of it have been [regularly] cited with evident approval in this jurisdiction”.

He further commented that the Law Reform Commission has drawn on it in formulating its recommendations for legislation enshrining the general duties of the expert witness in its Report on Consolidation and Reform of Aspects of the Law of Evidence.

He goes on to state, in the context of the matters at issue in that case:

1) “It is sufficient to set out what the authors of Hodgkinson and James, Expert Evidence: Law and Practice (5th edition, 2020) suggest is ‘clear law’ in both civil and criminal cases, as follows: (1) expert evidence presented to the court should be, and should be seen to be, the independent product of the expert uninfluenced as to form or content by the exigencies of litigation; (2) an expert witness should provide independent assistance to the court by way of objective unbiased opinion in relation to matters within their expertise. An expert witness should never assume the role of advocate.”

2) “In my opinion, these principles are ‘clear law’ in this jurisdiction also. Their essence is reflected in order 39, rule 57(1) of the Superior Courts Rules, providing as it does that ‘it is the duty of an expert to assist the court as to matters within his or her field of expertise. This duty overrides any obligation to any party paying the fee of the expert’.”

He concluded that, “where it appears that an expert is unable and/or unwilling to comply with his or her duty to give objective, impartial and independent evidence”, “then in my view their evidence should ordinarily be excluded as inadmissible”.

In his concluding comments, MacMenamin J, in dismissing the O’Leary appeal, applied the Ikarian tests when stating: “The evidence is not sufficient to conclude, in the words of Cresswell J in the Ikarian Reefer, that the testimony of the expert witnesses in this case was affected by ‘the exigencies of litigation’.

“Again, adopting the tests in the Ikarian Reefer, it has not been shown that the evidence was anything other than independent, objective and unbiased. No case has been advanced that there was some aspect of the evidence that fell below the range of duties identified in this, the main relevant legal authority cited to the court.

“There is, therefore, no sufficient basis for this court to conclude that the trial was unsatisfactory, or that the evidence was such as might have been rendered inadmissible. There is no sufficient evidence that would warrant a finding that, even had the facts regarding the charges been known, it would have affected the weight which the trial court would have attached to the evidence or its admissibility.”

Expert independence

The Supreme Court decision in Sweeney v VHI is also of interest, as it considers the Secretariat and the O’Leary decisions, and the fundamental importance of maintaining expert independence.

Experts should be fully cognisant of what Ferriter J in McLaughlin v Dealey describes as the Ikarian Reefer “classic statement of the duties of experts”, but experts should also remember that they are simply expressing an opinion as a skilled person.

The opinion should be objective, impartial, independent and unbiased, but it should also be clear, succinct and capable of being understood by a layperson.

The lawyer cannot effectively argue and be persuasive about the case unless he or she fully understands the technical evidence put forward by the expert, but the end goal of the expert should be to assist the court in reaching the correct conclusion.

The lawyer, in making the best use of the expert, needs to fully respect the duties, responsibilities and purpose of the expert in the process of endeavouring to persuade the court to favour their client’s case over that of their opposition.

Hands-on role

An expert should be hands-on in fulfilling this vital role and should be actively listening to the opposing evidence. The expert should also have a role at all stages in challenging the litigants’ and lawyers’ perceptions of the strengths of the case to bring some reality into the adjudication process.

Mark Tottenham BL, during above-mentioned 2021 webinar, described the expert witness’s primary duty as that of telling the truth. Mr Tottenham went on to identify the main duties of an expert witness, echoing the Ikarian Reefer principles.

He stated that the expert witness is required to:

  • Report, in oral evidence,
  • Be independent of the client and legal team,
  • Give evidence within their own expertise to assist the court in reaching its own decision,
  • Research or ascertain the relevant facts, to educate the court in specialist or technical knowledge,
  • Reach a reasoned and honestly held opinion, • Cooperate with the client and instructing legal team,
  • Cooperate with other legal teams when required, • Cooperate with other experts,
  • Communicate any change of mind,
  • Comply with the directions of the court.

Acid test

In order to make best use of the expert, the lawyer and experts need to be clear as to their respective roles.

The legal team needs to identify the relevant issues in dispute, and this must be confined to the issues between the parties. The expert should not be left to find the questions that need to be answered, and the legal team should not have a role in settling the conclusions of the expert.

The acid test is that everyone involved in the process should be satisfied that the conclusions of the expert would be no different, regardless of the party who has engaged him or her.

The court process seeks to get at the truth, and an objective, independent and unbiased expert can greatly assist the court in reaching the correct decision, provided that expert engages with the court and lawyers in a clear, succinct, and effective manner and applies the established principles.

The qualities required in an expert – over and above independence, impartiality, and having the requisite expertise – are probably the following: effective engagement, consistency, clarity and communication skills, confidence in presentation, concision, and high standards of preparation.

So, in overall summary, expert evidence is absolutely fundamental to the rule of law, as flawed expert evidence can lead to a court, acting in good faith, reaching an unsound decision. I have also articulated the classic guiding principles, duties and responsibilities of expert witnesses, as set out in the Ikarian Reefer case.

They have robustly stood the test of time and have been regularly cited with approval by the Irish superior courts, most recently in the O’Leary v Mercy, Sweeney v VHI, Duffy v McGee, McNamara v Revenue Commissioners, and McLaughlin v Dealey judgments.

The expert’s primary duty is simply to tell and present the truth in an independent, objective and unbiased manner, and his or her opinion should be based on relevant, accurate and complete information.

Brian McMullin is a member of the Law Society’s Litigation and ADR Committees, and is a member of the Council of the Law Society of Ireland.


The Ikarian Reefer case concerned the loss at sea of the Ikarian Reefer, which had run aground and then caught fire, leading to its abandonment off the coast of Sierra Leone in West Africa.

The question was whether those acts were deliberate or accidental – much depended on the expert evidence. The judge in the case, Cresswell J, was of the view that several of the expert witnesses misunderstood their duties and responsibilities, which meant that the trial took longer than he considered necessary (even allowing for the complexity of some of the evidence).

Therefore, he took the opportunity to set out what those duties were in what are now known as the ‘Ikarian Reefer principles’.




  • Superior Courts Rules, order 39, rule 57(1)


Brian McMullin
Brian McMullin is a member of the Law Society’s Litigation and ADR Committees, and is a member of the Council of the Law Society of Ireland