We rented a car from Hertz to get us around N Ireland in April 09. Picked up at Belfast International airport, and dropped it off there a couple of days later.
It's all very easy at a small airport like that. Took no time at all.
It was a Peugeot 207 (petrol) and underpowered compared to my own car. Got us there & back, however.
Cost £132 (inc vat) for 2 days.
BEWARE hidden charges - see my warnings tip!