The Suda's 'Sir', otherwise unattested, is evidently a foreshortened version of Sirmion/Sirmium (present-day Sremska Mitrovica in Serbia); Barrington Atlas map 21 grid B5.
[1] Originally settled by Illyrians (and Celts). Contrast the characterisation in Stephanus of Byzantium s.v.: 'a city of [the] Paionians'. NF: it is only 'Illyrian' in the wider Roman usage of Illyricum, as it lay in the later province of Pannonia, among the lands of the Skordiskoi/Scordisci.