The N21 route commences about southwest of Limerick city, just beyond Patrickswell. At the junction, which is reached by the main M20 motorway, the N20 diverges south to Cork and the main dual-carriageway becomes the N21 west. Prior to the 2001 opening the new dual-carriageway, traffic to Kerry left the city on the old N20 in a southwest direction on the Ballinacurra Road through Raheen and then went through Patrickswell to the beginning of the N21. The new M20/N21 dual-carriageway route begins at junction 1 on the Rosbrien Interchange as the M20 where it leaves the M7/N18 Limerick Southern Ring Road, and bypasses the Limerick suburbs of Dooradoyle, Raheen and Patrickswell, and continues, as the N21, almost to Adare. At this point the N21 becomes a standard two-lane road and flows through the main street of the scenic village. The N21 then runs by Croagh and Rathkeale, which were bypassed in 1986 and 1992 respectively, before entering the main county town of Newcastle West and filtering through the town. Further along the route the N21 passes through Templeglantine, before going through the town of Abbeyfeale. The road continues southwest, at the townland of Kilkinlea it crosses over the River Feale into County Kerry passing by Knocknagoshel and heading towards Castleisland. A new roadway from Abbeyfeale to Knocknagoshel, bypassing the accident-prone Headleys bridge near Knocknagoshel opened in July 2006. The remainder of the road to Castleisland was realigned in the 1980s. A section north of Casleisland was overlaid and brought up to standard in 1999. At Castleisland, the N21 heads west to Tralee. The short N23 continues southwest to Farranfore, allowing traffic to continue south along the N22 to Killarney. A new road from Castleisland to Ballycarty near Tralee was opened in early 2001. This was followed by a new section into Tralee, opened in April 2005. The N21 terminates at Camp Roundabout on the N22/N69 Tralee Bypass which opened in August 2013.
Recent upgrades
The Castleisland Bypass was the only major national road project starting construction in 2009. Construction work completed in October 2010. The new road was officially opened on Friday, October 22, 2010, by Minister for Defence Tony Killeen. The route consists of 5.3 km of type 2 dual carriageway and connects the N21 to the N23 bypassing Castleisland to the west of the town.
Barnagh realignments
A section west of Newcastle West was one of the more dangerous stretches of road in the country, known locally as Barnagh, between Newcastlewest and Templeglantine and once claiming 4 lives in 4 weeks, with 5 total deaths there in 2010. A local group known as the N21 Barnagh Road Realignment Petition campaigned to realign this section. The stretch of road was upgraded between 2011 and 2013.
Traffic congestion
The N21 is also known as a notorious traffic bottleneck particularly in the County Limerick towns of Abbeyfeale, Newcastlewest and Adare. The towns feature regularly on morning and evening traffic reports where delays of up to 30 minutes can occur. Local traffic, commuters and tourist traffic all contribute to the congestion.
Future upgrades
The Adare bypass is in the design phase and when finished, will carry the N21 around the village of Adare, Limerick. The proposed bypass is part of the ; near Rathkeale the N21 will connect with the proposed Foynes to Limerick road, which itself will bypass Adare and connect with the N20 en route to Limerick. A Rathkeale/Abbeyfeale road scheme, with a bypass of Newcastle West and Abbeyfeale was at the planning stage in 2006. As of October 2008 it has not advanced beyond planning stage.
