Seán started his football career as a youth with Cherry Orchard in Ireland and then Aston Villa before he played his first professional game with Longford Town. At youth level he also represented Ireland at Under-16 and Under-21 level, playing in the 2003 FIFA World Youth Championships. During three successful seasons at Longford, he won the FAI Cup (twice) and the League of Ireland Cup before joining Shelbourne in 2006, where he won the League of Ireland for the first time and also played his first game in European competition, in the Intertoto Cup.
In January 2007, Sean became Craig Levein’s second major signing, when he penned a two year deal at Tannadice. As a right-back, Seán became a regular in the starting line up and collected a Scottish League Cup runners-up medal as an unused substitute against Rangers in March 2008. That same season, Seán represented his country as a ‘B’ internationalist in a game against Scotland.
He retained his place in the side over the next three seasons, with his versatility making him a vial part of the team, playing at left-back, centre-half and centre-midfield as well as his usual right-back position. He made his 100th appearance for the Club against Hibs in January 2010, and in May that year he won a Scottish Cup winners’ medal against Ross County as United won the trophy for only the second time in the club’s history. A third place finish in the SPL for Peter Houston’s side saw Sean return to European football for the first time since his Shelbourne days.
During the 2010-11 season, Seán moved to a centre half role due to injuries to other players, and his performances in the heart of the defence won him acclaim amongst the fans, earning him the Federation of Supporters Clubs Player of the Year Award. In October 2012, Seán signed a three year contract extension and by early 2013, Seán had become the most featured non-Scot in the Club’s history – an appearance tally he continued to add to thereafter. Following the departure of his fellow countryman Jon Daly, he was named as the new Club Captain for the 2013-14 season under manager Jackie McNamara, however, with more competition for the centre half position, he moved in and out of the first team during the first half of the new campaign. However, he made his 250th appearance for the Club in an away match against Ross County in January 2014, by March he had re-affirmed his position in the team and in May, he collected a Scottish Cup runners-up medal against St Johnstone.
A League Cup runners-up medal followed during the 2014-15 season when United lost out to Celtic, and in May 2015, Seán signed a further contract extension that would keep him at the Club until May 2017 – making him the Club’s longest serving player since David Bowman. In October 2015 he hit the 300 appearance milestone, and in February 2016, entering his testimonial year, Seán became the first current player to be inducted in to the Dundee United Hall of Fame. However, the season ended in disaster as United finished bottom of the table and dropped to the Championship, but Seán was one of the few players kept on by new manager Ray McKinnon for the battle ahead.