The N3 is 79 miles long, and runs from Dublin city centre northwest to Cavan and on to the border with Northern Ireland. The route also connects from the Northern Ireland border at Belleek to Ballyshannon in County Donegal (the intermediate route through Northern Ireland follows the A509 to Enniskillen and A46 to Belleek). The N3, like many radial primary routes connecting Dublin to outlying parts of the island, commences as dual carriageway, with various route standards along later route sections, and poor two lane country route at the extreme in Donegal (Link Irish Roads).
Part of the old N3 route has been bypassed by the construction of 51 kilometres of new motorway. This stretch of motorway, designated M3, was opened on 4 June 2010. The M3 begins near the end of the dual carriageway outside Clonee and terminates south west of Kells just before the N52.