The vehicles used in the "Belfast" scenes have registration plates from the Irish Republic; however, Belfast is part of Northern Ireland and the U.K.
At the time of shooting, the rear plates of U.K. vehicles were generally yellow with black numbers/letters, and did not feature the E.U. flag, unlike vehicles registered in the Republic of Ireland, which are white with black letters/numbers, and do feature the E.U. flag.
It's true that the vehicles may have been rented in the Republic, and driven across the border (which at the time had no checkpoints), but the plates would have been a red flag for the Northern Irish police (PSNI).
It's true that the vehicles may have been rented in the Republic, and driven across the border (which at the time had no checkpoints), but the plates would have been a red flag for the Northern Irish police (PSNI).