Born in Ramelton, Ireland, Gallaher migrated to New Zealand with his family as a small child. After moving to Auckland, in 1895 he joined Ponsonby RFC and was selected for his province in 1896. In 1901–02 he served with the New Zealand Contingent in the Anglo-Boer War. He first appeared on the New Zealand national team for their unbeaten tour of Australia in 1903, and played in New Zealand's first ever Test match, against Australia in Sydney. The Originals Gallaher captained during 1905–06 helped to cement rugby as New Zealand's national sport, but he was relentlessly pilloried by the British press for his role as wing-forward. The use of a wing-forward, which critics felt was a tactic to deliberately obstruct opponents, contributed to decades of strain between the rugby authorities of New Zealand and the Home Nations; the International Rugby Football Board (IRFB) effectively outlawed the position in 1931.