Trespass on building, etc.
13.—(1) It shall be an offence for a person, without reasonable excuse, to trespass on any building or the curtilage thereof in such a manner as causes or is likely to cause fear in another person.
(2) (a) Where a member of the Garda Síochána finds a person in a place to which subsection (1) relates and suspects, with reasonable cause, that such person is or has been acting in a manner contrary to the provisions of that subsection, then the member may direct the person so suspected to do either or both of the following, that is to say:
(i) desist from acting in such a manner, and
(ii) leave immediately the vicinity of the place concerned in a peaceable or orderly manner.
(b) It shall be an offence for any person, without lawful authority or reasonable excuse, to fail to comply with a direction given by a member of the Garda Síochána under this section.
(3) (a) A person who is guilty of an offence under subsection (1) shall be liable on summary conviction to a fine not exceeding £1,000 or to imprisonment for a term not exceeding 12 months or to both.
(b) A person who is guilty of an offence under subsection (2) shall be liable on summary conviction to a fine not exceeding £500 or to imprisonment for a term not exceeding 6 months or to both.
Above is the wording of the relevant section of the relevant act - Sec 13 Criminal Justice (Public Order) Act.
There is no black and white definition of what will give rise to a prosecution.
Any prosecution of a complaint made will stand or fall on its own merits.