This article is about the Class III road numbered C666 maintained by DfI Roads. For other roads numbered C666, see C666.
The C666 runs from Omagh to Fintona in County Tyrone. It starts on the A32 in the south of Omagh and follows the former line of the A32 before it TOTSOs south westwards into the countryside. As the road meanders it passes to the west of Lough Muck and turns southwards, crossing the Ballynahatty Water. Soon the C663 is crossed and after a few kilometres, the road ends on the B46 in Fintona.