In 2004, McDonnell played for the New South Wales Under-17s team. The next season, he played for the New South Wales Under-19s team and made his NRL début for the Wests Tigers against the Penrith Panthers in Round 26 of the 2005 NRL season.
Wests Tigers
Having played one game for Tigers in 2005, McDonnell was named as the probable replacement for winger Pat Richards, if he was unable to overcome an ankle injury, to take his place in the 2005 NRL Grand Final team. In 2006, McDonnell played on the in the Tigers' 30-10 loss to Super League champions Bradford Bulls in the 2006 World Club Challenge, and also played for the New South Wales Under-19s team and Junior Kangaroos. in 2008 In 2008, McDonnell made himself available for Ireland's 2008 Rugby League World Cup campaign. Although named in the Ireland training squad for the World Cup, he was forced to withdraw through injury.
Newcastle Knights
On 7 May 2009, McDonnell joined the Newcastle Knights mid-season for the remainder of the year, after being released from his Tigers contract. At the conclusion of the 2011 NRL season, McDonnell joined Hull KR in the English Super League..
Hull Kingston Rovers
At the beginning of 2012, McDonnell joined the Hull Kingston Rovers in the Super League on a 1-year contract, following his former Knights assistant coach Craig Sandercock. In June 2012, McDonnell played for the Exiles.
Hull F.C.
On 26 October 2012, McDonnell signed a 2-year contract with Hull KR's cross city rival team in Hull FC, starting in 2013. In his first year at the club, he helped the team to the 2013 Challenge Cup Final against the Wigan Warriors where they were defeated 16-0.
St Helens
On 3 July 2014, McDonnell joined St. Helens mid-season for the remainder of the year, after resigning from his Hull F.C. contract. In October and November 2014, McDonnell played for Ireland in the 2014 European Cup. He scored two tries on début as Ireland beat France 22-12 in their European Cup opener. On 5 January 2015, McDonnell signed a train and trial contract to return to his first club Wests Tigers, in the hope of impressing for a second-tier contract. On 30 April 2015, McDonnell rejoined St. Helens effective immediately on a 2-year contract. In 2016, he was called up to the Ireland squad for the 2017 Rugby League World Cup European Pool B qualifiers.
