Treacy was first called up by Republic of Ireland U21 in November 2007. He made his U21 debut, starting the whole game, in a 1–0 loss against Montenegro U21 on 16 November 2007. However, Treacy suffered a hamstring injury in training ahead of the match and didn’t play for the Republic of Ireland U21 for the nine months. In August 2008, he was then called up to the U21 side and was given the captaincy for the first time. Treacy finally made his Ireland U21 debut on 20 August 2008, scoring in a 1–1 draw against Austria U21. After spending a year without the U21 side call–up following a fallen out with Don Givens, he was called up to Ireland U21 and made two more appearances by the end of 2009. Treacy went on to make nine appearances and scoring once for the U21 side.