The A3 is also the designated road number for the sections of the Irish N54 secondary route which lie in Northern Ireland.

The A3 begins by proceeding ahead at a crossroads in Lisburn city centre, where the A1 veers to the left.

Close to Portadown town centre, the A4 branches off towards Dungannon and Maghery, and the A3 continues to the city of Armagh, bypassing the village of Richhill.

It then continues its route through the villages of Milford and Middletown before reaching the border with the Republic of Ireland and County Monaghan in the townland of Ardgonnell.

The Monaghan–Cavan road resumes the A3 designation twice, where it enters southeast Fermanagh: first from the western outskirts of Clones for two miles, before reverting to the N54 in the Drummully salient in County Monaghan, part of the Republic; then again for another short stretch where it meets the B533 from Newtownbutler at a Y-junction at Wattlebridge before reverting again to the N54 until it meets the N3 at Butlersbridge, on the north-western edge of Cavan Town.