Youth Educated at
St Eunan's College in
Letterkenny, he warmed the bench during the College's 2000 McLarnon Cup victory.
Club Gallagher was part of the Glenswilly team that won the
2011 Donegal Senior Football Championship (his team's first County Championship at senior level). Glenswilly defeated
St Michael's by 1–8 to 0–9 in the final. He won his second Donegal Senior Football Championship with Glenswilly in
2013, scoring a goal in the final against
Na Cealla Beaga. The team had a successful Ulster campaign, advancing to the final of the 2013
Ulster Senior Club Football Championship, where they lost to
Ballinderry. He won a third Donegal SFC in 2016. He had retired by 2020.
Inter-county Gallagher was first called up to the senior team by
Brian McEniff for winter training in 2003. He made his senior debut for Donegal in 2004. That year his team made it to the Ulster final but were defeated by
Armagh. 2005 was unsuccessful. Donegal reached the 2006 Ulster Senior Football Championship Final and he played in that match at
Croke Park. In 2007, he was part of the Donegal team that won the county's first National Football League title. Donegal defeated
Mayo in the final. He was the caption that day. He sustained a heavy knock to the head, one that required a bandage, but was still able to collect the trophy. Alongside Glenswilly teammate
Ciaran Bonner, he was dropped by manager
John Joe Doherty over a breach of discipline ahead of the
2009 All-Ireland SFC qualifier game against
Carlow. By 2011, he had no career and was presumed finished. Then, quite suddenly, he became a linchpin of McGuinness's Donegal midfield, winning his first
Ulster Senior Football Championship in
2011. He won his second Ulster SFC title in 2012. Though he did not play in the final against
Down, he featured in earlier rounds and contributed a point in the quarter-final victory over
Derry. He was then part of the Donegal team that advanced through the
2012 All-Ireland Senior Football Championship. The best performance of his career with Donegal came against
Cork at
Croke Park in the All-Ireland semi-final; indeed, it is widely regarded as one of the all-time best in team history. He won an All Star and attended the Football Tour of New York. He also started for Donegal in the
2014 All-Ireland Senior Football Championship final. He won his third and final Ulster SFC title in 2014. Under the management of McGuinness's successor,
Rory Gallagher, he continued to feature for his county team. However, he was bedeviled by injuries. On
Valentine's Day in 2017, he attended training at
Convoy — it was upon the Convoy turf that he broke down for the final time and relinquished his status as an inter-county footballer. Gallagher later described Gallagher as "very disappointed… He wanted to give it a go… He got the back re-scanned and tried to build it up". ==Business venture==