From Wikipedia:
IrelandEdit
Any person can arrest someone who they have reasonable cause is in the act of committing or has committed an "arrestable" offence, that is one punishable by more than 5 years in prison.
[21] The arrest can only be effected if the arrestor has reasonable cause that the person will attempt to avoid apprehension by
Gardaí and the arrestor delivers the person to Garda custody as soon as is practicable.
In Ireland powers of arrest are to be found both at common law and in a large number of diverse statutory provisions. These powers for the most part are exercisable only by a member of the Garda Síochána. However, as the Deputy avers, there are situations where any person may effect what is known as a citizen's arrest. The basis for these important powers is set out below.
At common law all persons, including a member of the Garda Síochána, may arrest without warrant any person who has committed or is committing a breach of the peace in his or her presence or any person whom he or she reasonably believes is going to commit a breach of the peace in the immediate future.
The Criminal Law Act 1997 confers two distinct powers of arrest on any person including a member of the Garda Síochána regarding arrestable offences. Arrestable offences in general terms refer to offences which are punishable by imprisonment for five years or more and include an offence of attempting to commit such an offence.
Under section 4(1) of the 1997 Act any person may arrest without warrant anyone who is or whom he or she, with reasonable cause, suspects to be in the act of committing an arrestable offence. Section 4(2) provides that where an arrestable offence has been committed, any person may arrest without warrant anyone who is or whom he or she, with reasonable cause, suspects to be guilty of the offence.
Where either of these powers is exercised by a person other than a member of the Garda Síochána, two further qualifications apply: the power may only be exercised if the arrestor with reasonable cause, suspects that the person to be arrested by him or her would otherwise attempt to avoid, or is avoiding, arrest by a member of the Garda Síochána, and once an arrest has been effected by the ordinary citizen he or she must transfer the person arrested into the custody of the Garda Síochána as soon as possible. This was already the position with respect to the citizen's common law power of arrest.
Finally, under section 19 of the Criminal Law (Jurisdiction) Act 1976 any person may arrest a person whom he or she reasonably suspects of being in the act of committing, or having committed, any offence scheduled in that Act: these are offences of the kind associated with terrorism, if committed in Northern Ireland.