Mullally spent time on loan from Widnes at the Swinton Lions and Whitehaven in Championship 1.

[8] Mullally started his career at Championship side the Widnes Vikings in 2009 and was at the club when they won their licence to play in the Super League in 2012.

The end of his first season with West Yorkshire club saw him earn a place in Ireland's World Cup squad.

Mullally played in the 2017 Super League Grand Final victory over the Castleford Tigers at Old Trafford.

[9][10][11] On 20 September 2020, it was reported that he had signed for AS Carcassonne in the Elite One Championship[12] On 8 December 2021, it was reported that he had signed for Cornwall RLFC in the RFL League 1[13] Mullally was part of Ireland's 2013 Rugby League World Cup squad in which he featured in all three of their group games.