A member of Belvedere's U17 FAI Cup winning side in 2004, Niall signed for Bohemians in 2005. He made his League debut for Bohs on 11th August 2005 when he helped them to a 1-0 win over Bray Wanderers.

Shelbourne manager Dermot Keely signed Reilly from Bohemians in February 2007 and he made 15 league appearances before being released at the end of the season.

In 2008 he signed for St Patrick's Athletic but failed to make the breakthrough to the first team, moving on to Athlone Town for the 2009 season. He made 11 league appearances under Brendan Place and signed for Tony Cousins at Longford Town in February 2010. He made 27 league appearances over the following two seasons but in 2012 Niall made just 1 league appearance, and he was not a part of the club's 2013 squad.

In February 2013 Niall turned out for Bray Wanderers during their pre-season preparations but did not sign a contract with the club. Instead, Niall joined LSL side Tolka Rovers ahead of their 2013/14 season. He joined St Mochtas for the 2015/16 season.