Fine - so long as you have all your documents (driving licence and valid credit card in hirers name). We chose Hertz as they include cover to the the Republic of Ireland whereas Firefly (same company as Hertz but a bit cheaper) charge you £15 for crossing the border. Make sure that you take out separate excess cover to avoid being ripped off at the desk.
City/airport: Belfast international airport
Gunner's rating of their experience of Hertz:
(4/5) on 3 January 2020
Verified ReduceMyExcess customer