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The role of the expert witness

By Ms Stephanie O’Connell - 17th Dec 2023

expert witness

Ms Stephanie O’Connell outlines what to expect if you are called on to be an expert witness

Doctors are often asked to provide expert reports in legal cases and, if necessary, to give evidence in court, at inquests, or regulatory hearings in relation to those reports.

This article sets out the difference between an expert witness and a factual witness, explores the principal duties of an expert witness, and looks at some notable case law giving further guidance on the role.

What is an expert witness?

If a doctor is asked to prepare a report or give evidence in any proceedings, it is important to distinguish between giving evidence as a medical professional witness/factual witness and giving evidence as an expert witness.

A factual witness can give evidence concerning matters that they have personally witnessed, such as an examination of the patient in the course of treatment. They cannot give evidence of their opinion.

An expert witness, on the other hand, is an independent individual with no link to a case, who is retained to provide an expert opinion on matters outside the ordinary knowledge of the court. An expert witness can give evidence of what they know or believe to be true on the basis of their expertise.

The primary role of the expert witness is to provide the court with the necessary specialist knowledge to enable the court to apply that knowledge to the facts in issue and for the court to arrive at its own independent opinion on those facts.

Duties of an expert witness

Order 39 Rule 57 of the Superior Court Rules states: “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.”

It is crucial that any doctor acting as an expert witness does not to lose sight of the fact that their overriding duty is to the court, and not to the party instructing them. The Superior Court rules also require that experts acknowledge that duty by including a declaration to that effect in any report prepared by them. Expert witnesses are also obliged to disclose any financial or economic interest in any business or economic activity of the party retaining them.

Experts must ensure that their reports and evidence are confined to their area of expertise and should not give opinions on matters falling outside of their expertise. Experts should ensure that the evidence they provide in support of their opinion is accurate and not misleading. Reasonable steps should be taken to ensure that all relevant and available information has been considered. An expert who clearly states that they cannot comment on a particular issue, or acknowledges that a particular point could be interpreted differently by others, will usually enhance their credibility by doing so.

The new ethical guide

The Medical Council has provided guidance on the role and duties of an expert witness for the first time in the ninth edition of the Guide to Professional Conduct and Ethics for Registered Medical Practitioners, (the ‘Guide’), which was recently published. It will become operational on 1 January 2024. The Guide makes it very clear that the first duty of an expert witness acting in legal proceedings is to be of assistance to the relevant court or tribunal in providing an independent expert opinion.

The Guide sets out the following obligations when acting as an expert witness:

  • You must be honest and objective in all your spoken and written statements.
  • You must make clear the limits of your knowledge and competence.
  • You must not act as an expert witness in areas outside your scope of practice, experience, and expertise.

The Guide confirms that these obligations override any instructions from the person paying you a professional fee for your expert opinion.

The obligations set out in the Guide mirror the duties set out in the Superior Court Rules and case law. It is important for practitioners to bear in mind that any breach of the provisions of the Guide could result in allegations of poor professional performance or professional misconduct.

One important distinction between the approach to expert evidence in Ireland and the UK is that, in the UK, if an expert comments on a matter outside their expertise, their instructing solicitor can ask them to remove that comment from their report without impacting the expert’s duties to the court. There is no such equivalent provision in Ireland and any draft expert report provided by an expert to their instructing solicitor could be disclosed to the other side. For that reason, it is important that experts are clear on what their instructions are and are also able to resist any suggestion from instructing parties to amend their report, unless of course there are clear factual errors.

Case law

The duties of an expert witness have been addressed in several cases in the Irish courts.

In Duffy v McGee T/A McGee Insulation and GMS Insulations Limited, a Court of Appeal judgment from November 2022, it was held that: “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. An essential component of the duty of the expert is to ascertain all relevant facts whether they support the client’s case or not.”

The expert in this case was a toxicologist, but had strayed far beyond his remit by commenting on other matters. The Court of Appeal held that: “Far too frequently, expert witnesses appear to fundamentally misunderstand their role and wrongly regard themselves as advocates for the cause of the party by whom they have been retained.”

The Court identified a number of ‘red flags’ in the expert’s evidence including:

  • Expressing views on legal doctrines when acting as a toxicologist.
  • Making categorical statements about disputed issues about which he had no independent knowledge.
  • Accusing the plaintiffs of misrepresentation.
  • Accusing the plaintiffs of not telling their treating doctors the full truth and criticising their doctor’s evidence as a result.
  • Expressing views on reports prepared by a psychiatrist.

This judgment serves as an important reminder to expert witnesses and legal practitioners alike of the strict parameters of the role and responsibilities of an expert witness when engaged in litigation.

In the 2015 High Court case of Waliszewski v McArthur, a medical expert failed to mention in his report that the plaintiff had been involved in a subsequent road traffic accident as he thought this might cause confusion.

The Court stated that “…his failure to do so is reprehensible and is to be deprecated” and rejected the expert’s explanation that the omission was with the intention of not causing confusion. It is noteworthy that the Court in that case directed that a copy of its decision be sent to the Medical Council.


Barriers to undertaking expert witness work, including time constraints and unfamiliarity with the legal system, mean that doctors who take on expert work are often those at the end of their careers, some of whom have been out of clinical practice for a considerable time.

Doctors might also be discouraged from acting as an expert witness in a case involving a colleague and this has traditionally been a difficulty in Ireland given the relatively small size of the medical community. Doctors should be reassured that knowledge of a party to litigation or inquiry does not necessarily disqualify them from acting, but they should be careful to disclose any connection that they might have with a case or party to it.

There is arguably a benefit to the profession, and indeed the administration of justice, if Irish experts can give evidence in Irish courts. A local expert, who is also a practising doctor, will have a keen understanding of the workings, and the nuances, of the Irish healthcare system.


It is important that there are enough medical practitioners, across all disciplines of medicine, that are willing and able to act as expert witnesses. The new provisions in the Medical Council’s Guide provide very clear guidance to doctors considering taking on this important work. Doctors who act as expert witnesses should familiarise themselves with the obligations and duties associated with the role to ensure that they discharge their duty to the court appropriately and should bear in mind: “The acid test is whether the expert’s opinion would not change regardless of which party retained him or her.” (Supreme Court of Canada – White Burgess Langille Inman v Abbott and Haliburton).

References available on request

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