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 road continues southwest, at the townland of Kilkinlea it crosses over the River Feale into County Kerry passing by Knocknagoshel and heading towards Castleisland.

The Castleisland Bypass was a national road project which commenced in 2009 and for which construction work completed in October 2010.

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.

The towns (most notably Adare) feature regularly on morning and evening traffic reports where delays of up to 30 minutes can occur.

[citation needed] Originally proposed as part of the Foynes to Limerick Road Improvement Scheme,[5][6] a 33 km (21 mi) bypass of Newcastle West and Abbeyfeale was at the planning stage in 2006.

[citation needed] Updated bypass plans were subject to additional inquiries by An Bord Pleanala in early 2021.