See also: Irenaeus (disambiguation). Saint Irenaeus of Sirmium (died 304 AD) was an Illyrian bishop of Sirmium in Pannonia, which is now Sremska Mitrovica, Serbia.[1] He was bishop during the reign of Diocletian. He refused to offer pagan sacrifices, even at the behest of his family. He was condemned by the governor of Pannonia, and then beheaded on 24 March 304.