The N18 road is a national primary road in Ireland, connecting the cities of Limerick and Galway. Ennis and Gort are two major towns on the route, and Shannon Airport is reached via the connecting N19. The west coast route continues on to Sligo as the N17. The section from Limerick to the N19 junction forms part of European route E20, and the entire route is part of the proposed Atlantic Corridor. The section of motorway from Shannon to Claregalway is designated the M18 motorway. The tolled Limerick Tunnel forms part of the route.

A national primary road is a road classification in Ireland. National primary roads form the major routes between the major urban centres. There are 2649 km of national primary roads. This category of road has the prefix "N" followed by one or two digits. Motorways are prefixed "M" followed by one or two digits.