Recommended:
Jurys Inn - City Centre, £78 per twin room | Directions | Map
Express By Holiday Inn - near City Centre, £59 per twin room | Map
Park Avenue Hotel - near Belfast City Airport, £95 per twin room | Map
Park Plaza Hotel - beside Belfast International Airport, £35 single room, £60 twin room | Map

Budget:
Belfast International Youth Hostel 22-32 Donegal Road, near Queen's University.
5 minutes walk from the the Europa bus station, where the Dublin bus stops. If arriving by train from Dublin, take the connecting train to Botanic station, there is no need to buy another ticket. £10.50 per person in 4-bed dorm.

Alternative:
A range of accommodation can be found at the following links:
Discover Northern Ireland
GotoBelfast.com