Acting as an Expert Witness requires more than just particular knowledge in your specialised field. It requires full knowledge of your role and duty to the Court, understanding what this is; what burden it places on you; how to discharge this duty; and what the courts expects and need from you.
Recent case law in both Ireland and the UK has highlighted the importance of an Expert Witness being suitably qualified and understanding their role and duty to the Court. As an expert, you must be prepared for your qualifications and opinions to be closely scrutinised and challenged in the Courts. If these qualifications and opinions do not stand up to this scrutiny and challenge, there is every possibility you will be criticised by the presiding Judge. This is well demonstrated in these recent judgements.
In the Irish High Court judgement of McKillen v Tynan  IEHC 189 judicial review proceedings were dismissed on certain grounds, including that the Applicant’s two expert witnesses were not qualified to give evidence as experts; and even if they were qualified, the expert witnesses were not suitably independent, their evidence had no weight given their patent desire to advance a case made by the Applicant.
In the UK Decision Farah v Abdullah & Others  EWHC 825 (QB), expert evidence was scrutinised and the Judge was both complimentary and critical of different expert evidence in this case.
Extract from the judgement:
I found [Expert 1] to be a highly reliable witness whose evidence I accepted. I did not accept [Expert 2’s] evidence on the key issues in this case, not only because it did not accord with my view of the evidence, and that of the other experts in the case, but also because I found him to be highly unreliable as a witness. For the avoidance of doubt, I am not in a position to comment on his qualities as a doctor and do not do so. But, for reasons which I will explain below, I found his approach as a witness to be careless and partisan in a way which was inconsistent with his role and duties as an expert.
In another UK Judgement in December 2020 Z v (1) University Hospitals Plymouth NHS Trust, (2) RS (by his Litigation Friend the Official Solicitor) & Others. The judge stated that he did not think he could put any weight on the expert’s evidence.
The full reasoning is given at paragraphs 13 to 30 of the judgement. The main issue identified by the judge, however, was not only that the expert based his opinion on a lack of evidence, but that the expert failed to note the lack of evidence and to accept that this placed limitations on his opinion. The expert was under a duty to express his opinions as qualified opinions and to accept, and refer to, those qualifications.