N15

The N15 commences halfway across Lifford Bridge (which crosses the River Foyle on the border between the Republic and Northern Ireland), continuing the route of A38 (from Strabane on the east side of the river). In Lifford, west of the River Foyle, the N14 meets the N15, ending at a junction in the town centre. The N15 leaves to the southwest, changing to run west at the point just west of Clady across the border. It continues west through Castlefin, Liscooley and Killygordon to reach Stranorlar.

In Stranorlar, the N13 commences, leaving the N15 to head north, with the N15 itself turning southwest to cross the River Finn to enter Ballybofey. From here, it continues southwest through the mountains and Barnesmore Gap (passing southeast of Bluestack Mountain) towards Donegal Town. East of the town itself, the N56 road starts at a junction on the N15 and runs around the north of the town, with the N15 turning to the south, to bypass the eastern side of Donegal Town.

South of Donegal Town, the N15 passes by Ballintra to the west. Further south, the route follows a road opened in 2006, bypassing Ballyshannon to the east and crossing the River Erne on a viaduct. Bundoran, southwest of Ballyshannon, marks the last town in County Donegal that the N15 passes by; it enters County Leitrim directly west of Bundoran (the bypass passes south of the town). Tullaghan is the only town along the N15 in Leitrim, as the route passes into County Sligo shortly after, having run through Leitrim for just a few kilometres.

The N15 runs closer to the coastline in County Sligo, passing through Castlegal, Cliffoney and Grange as it proceeds southwest. After Grange, the route veers south, running further away from the coast as it passes through Cashelgarran. From here the route continues south, in the shadow of Benbulbin, through Drumcliffe (where Drumcliffe Bay brings the coast closer to the road again).

Sligo itself is entered from the north. The route ends at the start of the N4, which continues through the town as the Sligo Inner Relief Road dual carriageway.