The N15 commences halfway across Lifford Bridge, continuing the route of the A38. 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. 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 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. The N15 crosses the Drummenny Burn on the eastern outskirts 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. The road enters County Leitrim in the west of Ireland, directly west of Bundoran. Tullaghan is the only village along the N15 in the county, also bypassed. The N15 runs closer to the coastline in County Sligo, passing through Cliffoney and Grange as it proceeds southwest. The road then passes through the villages of Drumcliff and Rathcormack before entering Sligo. The route ends at the start of the N4, which continues through the town as the Sligo Inner Relief Roaddual carriageway.
Planned upgrades
The route between Lifford and Stranorlar is a single carriageway road of very poor quality. A replacement of the current alignment with a new single carriageway road is planned, bypassing Castlefin, Liscooley, and Killygordon. This will connect with a 2+1 bypass of Stranorlar and Ballybofey which is also planned. This will bypass the towns to the south, with a new crossing of the River Finn. The N15 runs for over in County Sligo, passing north of the mountains and around Benbulbin before turning around Drumcliffe Bay to enter Sligo Town. A bypass of the entire route was planned, with dual carriageway for at least part of the route. A route had been selected for the project, with the new road to run closer to the mountains and the bay than the existing N15. However, the NRA didn't give funding for the road and the project was suspended The localradio station in the North-West, Ocean FM had a special report in 2017 on how the road in Sligo needs to be upgraded in the wake of 3 crashes on this stretch of road in less than a fortnight. A major repaving of the road from Creevykeel Crossroads to Castelgal was started in mid-October and were very nearly complete before the Covid-19 lockdown happened in Ireland. They returned in late May to put the finishing touches on the road. The road improvements included Lowering the speed limit at Creevykeel Crossroads to 60 km/h either side of the crossroads. Despite the road improvements, The road from Castelgal to The Sligo county boundary was still in bad condition with locals calling for the whole road to be repaved.
