In some of the more local pubs they won't id you unless you look particularly underage.
The ones where you will get id'd you can tell by the guys outside asking for id :-/ Although even then, if you look confident enough or you go in early enough, you can get into the pub and probably not be id'd all night
I wouldn't recommend trying it in the union though, if it's quiet enough then they send someone around to watch you buy your drink after checking your matric card.
And if you can't be bothered with all that, then just find out where all the parties are, get older friends to buy your alcohol from tesco and go get drunk at someone's house

"This is my story. It'll go the way I want, or I'll end it here"
--Final Fantasy X