It starts at a roundabout at Ballymascanlan, just north of Dundalk, where it meets the N52 and the M1/N1 roads.

It then heads east, along the northern coast of Dundalk Bay to The Bush where it veers north, past the village of Grange to Carlingford.

From there it heads NW along the southern shore of Carlingford Lough, through the village of Omeath to the Northern Ireland border where it becomes the B79 into Newry, County Down.

The R173 is a former Trunk Road and was previously numbered as the T62.

In June 2024, construction began on the Narrow Water bridge connecting the road to the A2 road in County Down, under the Shared Island scheme.