Running through the Irish Midlands from Cavan to Athlone, the N55 provides a local link between the N3, N4, and N6.
The route starts at Dublin Road Roundabout on the N3 Cavan bypass, some 2 km east of the town centre and follows a south-south-easterly course to Granard in County Longford. In August 2013 a new 2.7-km-long alignment was opened between Mullaghoran and Dundavan northeast of Granard.
From Granard the N55 continues on an almost perfectly straight alignment (although at the expense of a number of blind summits and hidden dips) to meet the N4 at a roundabout junction (Maria Edgeworth Roundabout) on the southern edge of Edgeworthstown. After following the N4 – the town's bypass – for some 450 m northwestward to Goldsmith Roundabout, the N55 resumes its old course along Ballymahon Road.
Ballymahon itself is reached 19 km later, and here the route turns left to follow the town's Main Street for 350 m, before crossing the River Inny and making a sharp right turn into Athlone Road. The final section leads for 22 km down to the N6 Athlone bypass at Cloghanboy, a kilometre or so northeast of the town centre.