Irwin is a borough in Westmoreland County, Pennsylvania, 22 miles (35 km) southeast of Pittsburgh. Some of the most extensive bituminous coal deposits in the commonwealth 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.

Westmoreland County is a county in the Commonwealth of Pennsylvania, United States, located in the Pittsburgh Metropolitan Statistical Area. As of the 2020 census, the population was 354,663. The county seat is Greensburg and the most populous community is Hempfield Township. It is named after Westmorland, a historic county of England. The county is part of the Southwest Pennsylvania region of the state.