Newcastlewest used to be called Castle-Roe, after a castle built there by the Knights Templars in 1184. The town then came to be known as Newcastle, West Limerick, but over time the 'west' became part of Newcastle and the town was known by the current name Newcastlewest.
According to Begley, the parish was called Newcastle and Ardagh in 1704. Newcastle was joined with Monagea from 1722 until 1764 when it became a separate parish. Lewis stated that parts of Monegay and Killeedy were in the parish of Newcastlewest.
Two notable mentions in the history of the diocese were the first Monsignor in the diocese, Richard Baptist O'Brien in 1881, and Denis Hallinan who was P.P. here and later became Bishop.
Sir William Courtenay, the local landlord, held 10,500 acres of land in Newcastlewest in the late 16th century. He was a staunch Catholic, and suffered persecution for his beliefs. His son George may have practiced his faith in secret. Their home was reputed to have had a room in which priests were hidden. William Courtenay was denounced in the House of Commons as a papist recusant in 1624.
During the reign of Elizabeth I, three battles were fought near here. Tradition has it that the locals killed many of the Knights Templars. The town was sacked in 1302 and destroyed in 1315. Two of the Earls of Desmond died here. Garrett (better known as Gearóid Iarla) in 1399, and James, the 8th earl, in 1462.
Markets were held on Tuesday, Thursday, and Saturday. Large numbers used to attend Thursday's hiring fairs for farm workers. Fairs were held on April 1st, May 3rd, July 12th, August 20th, October 1st, and December 10th.
The motto that goes with the town coat of arms is "As Dúchas Dóchas", which may be translated as 'Our Hope springs from our Traditions'. The town has a population of around 3,500.