Byrne first came to prominence on the inter-county scene as a member of the Offaly senior hurling team. He made his debut in a National Hurling League game against Antrim in 1998. Later that year he made his championship debut against Meath before lining out in his first provincial decider. Kilkenny provided the opposition on that occasion, however, a 3-10 to 1-11 defeat was Byrne's lot on that occasion. This defeat prompted the Offaly manager, Babs Keating, to describe the team as "sheep in a heap", and he promptly resigned. It looked as if Offaly's championship hopes were in disarray, however, they overcame Antrim in the All-Ireland quarter-final and qualified to meet Clare in the semi-final. The first game ended in a draw and had to be replayed, however, the replay was ended early because of a time-keeping error by the referee Jimmy Cooney. Following a protest on the pitch of Croke Park by the Offaly supporters it was decided that Clare and Offaly would meet for a third time. Byrne's side won the third game and qualified to play Kilkenny in the final. On that day Brian Whelahan delivered one of his greatest-ever Offaly performances, scoring 1-6. Offaly reversed the Leinster final defeat by winning the All-Ireland final by 2-13 to 1-16. It was Byrne's first All-Ireland medal. He was later honoured with an All-Star award while he was also named Eircell Young Hurler of the Year.