Kirby returned to Ireland and signed for Waterford United in January 1999, and made his League of Ireland debut against Bohemians at the Waterford Regional Sports Centre on 29 January. His first senior goal opened the scoring as Waterford beat Dundalk 2–0 on 16 April, and his second, a "glorious 20-yard volley that simply flew past" the goalkeeper, was the only goal of the game against fellow relegation candidates Bray Wanderers a few days later and made a significant contribution to Waterford retaining their Premier Division status.