We have prepared a guide for those who are to attend court as witnesses here.
There are some excellent Youtube videos which are available. Although they are prepared for a U.S. audience, these guides are very much applicable to anyone preparing to attend court in order to give evidence in Ireland. Note, the vast majority of civil cases in Ireland will have no jury, and the judge will decide all issues of law and fact without the involvement of a jury.
Howcast.com - Helpful guide to giving testimony in Court.
Craig Penney - The Holy Trinity of Testimony
Ryan Foley - Body Language Tips for Testifying in Court