check out the premier inns - they have 2 hotels in belfast - cheap and clean. http://www.premierinn.com/en/home.action
the one in waring street has no onsite parking, not sure about the other one.
Might not be quite what you are looking for but the [broken link removed] does apartments. I found them both reasonable and comfortable. Have stayed in the Mews apartments twice.
Got a good deal on the culloden hotel 2 years ago. 2 interconnecting rooms (for the price of one)/ Also stayed in premier inn. Staff were lovely and hotel clean etc but not the same amount of room.
Might not be quite what you are looking for but the [broken link removed] does apartments. I found them both reasonable and comfortable. Have stayed in the Mews apartments twice.