On 25 May 2010, Fahey won his first senior cap for the Republic of Ireland in a 2–1 win against Paraguay at the RDS Arena, replacing Damien Duff in the 77th minute. He made his first start against Argentina on 12 August, in the first international match at the new Aviva Stadium. Fahey scored his first goal for the Republic of Ireland eight minutes into his competitive debut, coming off the bench to score the only goal in a Euro 2012 qualifying win over Armenia in Yerevan on 3 September 2010. On his first competitive start, in a 1–1 draw against Slovakia in Žilina on 12 October, Fahey delivered the free kick that led to Ireland's goal, scored by Sean St. Ledger. In February 2011, he scored his second goal for Ireland in a 3–0 defeat of Wales in the 2011 Nations Cup, and his third came in March 2011, a penalty against Uruguay.