Competence of a witness
The general rule in Irish law regarding competence of witnesses is that all human beings are competent witnesses. In other words, everyone is capable of giving evidence in a case. However, there are some exceptions.
Sometimes two or more people may be charged with a criminal offence and they are tried at the same time. In that situation, the people being tried are called "co-accuseds" and the rule is that one co-accused cannot be called by the prosecution to give evidence against another co-accused.
The spouse of the accused
Originally the spouse of an accused was not competent to testify for either side in a case. However, the law has now changed and a spouse may give evidence for either the defence or the prosecution (unless he or she happens to be a co-accused). He or she may only be forced to testify for the prosecution against his or her spouse in certain circumstances.
Originally children could only give evidence on oath and in order to take the oath, the child had to understand the nature and consequences of the oath.
Since the introduction of Sections 27 and 28 of the Criminal Evidence Act 1992 for criminal proceedings and Section 28 of the Children Act 1997 for civil proceedings, any court can accept evidence from a child under 14 years of age without the child taking the oath. The court must be satisfied that the child is capable of giving an intelligible account of what he/she witnessed. Even though such evidence is not on oath, the child is guilty of an offence if he/she knowingly gives evidence which is false or untrue.
In cases where a child under 14 years is offered as a witness by the prosecution, it is not unusual for the judge to request to talk to the child in private before the child gives evidence, in order to assess the child's ability and capacity to give an intelligible account of events. This normally takes the form of a general conversation with the child but does not involve the judge asking questions about the evidence the child is about to give.
Children over 14 years of age must still make the oath or affirmation in order to give evidence.
People with mental disabilities
People with mental disabilities are dealt with in the exact same way as children under 14 years of age. Under the Criminal Evidence Act 1992 and the Children Act 1997 if a court is satisfied that a person with a mental disability is capable of giving an intelligible account of the events which are relevant to the proceedings then the court may receive that evidence from the person without the oath.
For more detailed information you should seek legal advice.