I believe it's actually two small tickles, if memory serves, both from her older boyfriend in the movie (not the Cillian Murphy character). The second one is mainly off-screen, from memory, but involves her attempting to have a serious conversation with him when he doesn't want to listen and starts tickling. The second one goes on a bit longer than the first.
It was enough that the girl I saw it with jokingly called me a 'dirty old man' for tickling her after seeing that scene, anyway. But I did see it when it first came out, so details might be a bit sketchy in my mind. Worth having a look at anyway, and it's a pretty good Irish movie apart from that.
