Irwin is a borough in Westmoreland County, Pennsylvania, United States. The population was 3,902 at the 2020 census. Located 22 miles (35 km) southeast of Pittsburgh, some of the most extensive bituminous coal deposits in the commonwealth are located in Irwin. In the past, iron foundries, flour mills, car shops, facing and planing mills, electrical goods and mirror factories were based in the borough.