Fahey began her career with her hometown club, Salthill Devon. She progressed through the ranks alongside future international team mate Méabh De Búrca. Both players helped Galway Ladies (a regional select team) win the FAI Senior Women's Cup in 2007. In the final against Raheny United, Fahey scored the winning penalty and also subdued the attacking threat of Raheny's Olivia O'Toole, in what RTÉ described as "a splendid individual display."
She joined Arsenal Ladies in August 2008. In her first season with the Gunners Fahey made 18 appearances, mostly at left full–back, as the club won a domestic treble.
Fahey has won over 25 caps for Republic of Ireland, having previously represented her country at Under–17 and Under–19 level.
In March 2007, Fahey made her senior debut for Ireland in a 1–1 draw with Portugal in the opening match of the Algarve Cup. After becoming a regular in the team, Fahey was named FAI Women's Senior International Player of the Year in 2008, 2009 and 2011.