Irwin is an unincorporated community in southwestern Union Township, Union County, Ohio, United States. It is located at 40°07′21″N 83°29′22″W / 40.1225°N 83.48944°W / 40.1225; -83.48944Coordinates: 40°07′21″N 83°29′22″W / 40.1225°N 83.48944°W / 40.1225; -83.48944, at the intersection of Ohio State Route 4 and 161. The Irwin Post Office was established on 15 May 1858, and serves the surrounding area, including the community of Rosedale.