Me and a friend are flying to N.Ireland in June to see Deadmau5.

Is a provisional driving license sufficient identification? or would we need a passport, if so can we used expired passports?
A provisional license is photo ID and still a type of driving license and should be accepted. You do no need a passport if you are within the UK BUT you'll have to check what ID is accepted by the airline you are using. They all have different ID criteria. Contact them if you're still unsure.
There's a good chance that what I've written above is useless and if you take any of the advice it's your own fault.