The R392 runs from Marlinstown roundabout on the N52 eastern bypass of Mullingar in County Westmeath, through Mullingar town centre, and then along a west-by-northwesterly course which takes it into County Longford and to meetings with, first, the N55 – in Ballymahon – and finally the N63 at Lanesborough on the left bank of the River Shannon.
The road is characterized by its many long straight stretches across low-lying countryside, with relatively good agricultural land near Mullingar becoming cutaway bog further west. Even the very few tight bends and corners which once existed along the route have now been replaced by much gentler curves, although there are some dips in the route restricting visibility. The Bord Na Mona bog railway crosses the R392 at a level crossing 8 km south east of Lanesborough.
The major tourist attraction along the route is the museum for the Corlea Trackway an Iron Age wooden Corduroy road laid around 148 BC. The 3.5 m wide trackway runs about 1.5 km north of the R392. Historians are unclear whether this elaborate road served a transport purpose, perhaps leading towards the same Shannon crossing, or whether its role was mainly ceremonial. In any case, the road was not much used and become quickly covered over by bog.
Prior to the Classification of Regional Roads Order 2012, the first 5 km of the present course of the R392 through Mullingar from Marlinstown to Grange South had been a part of the R156 and the R390.