The A509 is the southeastern portion of the N3 route through Northern Ireland and forms part of the T10 trunk road. The road was originally the southern half of the B37 and gained its present number in the 1960s.
A couple of kilometres north of Belturbet the N3 from Dublin crosses the border at the George Mitchell Bridge over the Woodford River to become the A509. The old bridge just downstream from here was blown up in 1972; as such the A509 started in the middle of nowhere before it was connected to the N3 in 1999.
The road heads northwest past Killymackan Lough before continuing across open country. The road goes through Teemore to reach Derrylin. On the near side of town there is a double mini-roundabout with the B127 whilst on the far side the road turns sharply right at its junction with the B108. The road continues on, crossing a number of the rivers leading into Upper Lough Erne, which is over to the right. Presently, after avoiding a loop of the River Erne, the road reaches the outskirts of Enniskillen. It ends on the western side of town at a roundabout on the A4.