I think the point though is that this tenant had originally agreed to stay until May. I've been in similar situations where I held the lease with one other person on a house and we sub-leased the two other rooms in the house. It happened a couple of times that people moved in promising to stay at least a year and then moved out six or eight months later (often leaving in May, when it was most difficult to rent to anyone as we lived near a college and loads of rooms became available in the area around then). It's very frustrating when you take people at their word - at the very least if your agreement was one month's notice and he has paid only for two weeks of that month you should get the remainder of the rent for the notice period. However, a lot depends on whether you had a written or verbal agreement and what was stated in it regarding deposit and notice period and also on how your relationship with the tenant was.
In the meantime, have you advertised on DAFT etc.? You might be lucky and get someone who wants to move straight away, in which case, if he's moved out already but still paid up to the end of the month, you might end up making a profit.
I'd also recommend contacting Threshold and the Private Tenants Rental Board (is that what it's called?).