Irwin is a borough in Westmoreland County, Pennsylvania, southeast of Pittsburgh. Some of the most extensive bituminous coal deposits in the State are located here. In the past, iron foundries, flour mills, car shops, facing and planing mills, electricals goods, and mirror factories provided employment to the residents. In 1900, the population numbered 2,452; it increased to 2,886 in 1910. The population was 3,973 at the 2010 census.