The N12 road is one of the shortest national primary roads in the Republic of Ireland, running 6.87 km (4.27 mi) from outside Monaghan (where it leaves the N2) to the border with County Armagh, Northern Ireland, at Ardgonnell Bridge outside Middletown.

The overall Cavan Town-Belfast route, of which the N12 is but a link, passes through Butlersbridge, Wattlebridge, Clones, Smithborough and Monaghan Town as the N54 / A3.

A small aqueduct runs parallel to Ardgonnell Bridge, where the N12 meets the A3.

The N12 was formerly part of the T15 trunk road, which was a route from Galway to the border (and eventually Belfast).

However, Monaghan County Council include a proposed realignment of the route in their development plan.