The former L16 was a long meandering route from the coast west of Sligo, through the town and past Lough Gill before turning north. After short multiplex with the T17 (now N16) to Manorhamilton, it continued north to Lough Melvin and the border. It crossed the border twice north of Pettigo before joining the B166 towards Castlederg, County Tyrone. It then returned to the Republic as the road between Castlederg and Castlefinn. Crossing into Northern Ireland at four locations, it is probably the road number that crossed the border most frequently.