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. The N15 crosses the Drummenny Burn on the eastern outskirts of Donegal Town.