As far as I am aware the rules for trespass are the same in Northern Ireland as they are in England and Wales. Scotland is different.
If you do no damage you are not committing a criminal offence and cannot be prosecuted.
You are, however, committing a civil offence simply by going on land belonging to another without the owner's permission. He can take you to court and get an injunction to stop you doing it again.
Some forms of trespass are criminal offences - such as those involving travellers, gypsies, hunt saboteurs and raves.
Here is a good starting point:
http://www.desktoplawyer.co.uk/dt/browse/law/i ndex.cfm?fuseaction=ViewContent&sid=75920&aid= 34460
http://www.lawteacher.net/Trespass%20to%20Land .php