|Distance:||84.4 km (52.4 miles)|
|Meets:||M20, N20, R256, R523, R518, R521, R522, R520, R515, R524, R576, N23, R577, N69, R875, N22, N70|
The route starts as a continuation of the M20 mainline coming out of Limerick, and narrows back down to a single carriageway before passing through Adare. It follows the original course of the road, before a single carriageway bypass of Rathkeale, before heading through Newcastle West and Abbeyfeale, and passing into County Kerry.
From here to Castleisland, the road is, again an offline single-carriageway road away from the original route, up to a roundabout halfway along the Castleisland Bypass with the N23. The N21 then turns right towards Tralee. Heading along a new route opened in 2005 and ends at the Camp roundabout junction with the N22 and N70 on the Tralee bypass. The old road into Tralee (from the Ballingowan Roundabout) has been renumbered as the R875