Buuxi weedha soo socota: Wuxuu ku dhashay Cobh, County Cork, O'Sullivan wuxuu wax ku bartay Cobh Community College. Waxay dhamaysatay shahaadadeedii qalinjabinta sanadkii 1987. O'Sullivan tartankiisii ugu horeeyay ee caalami ah wuxuu ahaa
1990 Horyaalka Yurub ee Split, halkaas oo ay ku dhammeysay kaalinta 11-aad ee 3000 m. Si kastaba ha noqotee, O'Sullivan si dhakhso leh ayey u fiicnaatay ka dib taas, iyo 26kii Janaayo ee sanadka soo socda, magaalada Boston, waxay rikoor cusub oo adduunka ah ka dhigtay gudaha gudaha 5000 m ee 15:17.28, iyadoo ka qaadatay in ka badan 5 ilbiriqsi rikoorkii hore. Waqtigaas, O'Sullivan (oo heshay deeq waxbarasho isboorti) waxay ka baraneysay xisaabaadka Jaamacadda Villanova ee Mareykanka, iyo Ciyaarihii Ardayda Adduunka ee Sheffield bishii Luulyo 1991, waxay ku guuleysatay Bilad Dahab ah 1500 m 4:12.14, iyo billad dahab ah 3000 m 8:56.55. Bilowgii 1992, waxay ku dhammeysay kaalinta 7-aad ee Tartanka Adduunka ee Cross Country.