The N21 road is a national primary road in Ireland. The route runs from the M20 outside Limerick to Tralee with connecting roads to other parts of County Kerry. It is 84.37 km (52.43 mi) in length. It runs through the towns of Abbeyfeale, Newcastle West, Adare and the village of Templeglantine. Prior to October 2010 the N21 also ran through the town of Castleisland. The town has since been bypassed. Rathkeale was also bypassed in 1992.

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.