Commissioner Irwin was first elected to the Washtenaw County Board of Commissioners in August 1999. Commissioner Irwin is the 2007 Board Chair.  Commissioner Irwin has previously served as the chair of the Board Working Session and Ways & Means Committee.

Commissioner Irwin is a member of the Brownfield Redevelopment Authority, Drug Forfeiture Committee, Economic Development Corporation, Intergovernmental Partnership Committee for Gelman Groundwater Remediation Plan, Money Purchase Pension Plan, Public Works Board, Sheriff's Community Relations Advisory Board Ad Hoc Committee, Solid Waste Management Consortium, and Southeast Michigan Council of Governments (alternate).