In Dublin slang it means a night of mayhem. Gaff means house, therefore by the end of the night of drinking/taking drugs/ talking rubbish to people you don't know, the house or flat will have been destroyed. You must say it in a grating working class Dublin accent for full effect. Since Dublin clubs close ridiculously early, people are forced to gather in someone's house or flat (often someone they don't know at all but that's not important) and drink and take drugs until a) everything is gone b) they pass out or sometimes c) wait until the off licences open the next day so they can start wrecking the gaff even more.
It was a great night, we wrecked the gaff.
We went to some bird's house in Tallaght, totally wrecked the gaff.
Come over, we're gonna wreck the gaff!