Age: 35 years, 3 months
Place of Birth: Drogheda
Sean is a brother of Ryan, Gavin and Killian who also play in the League of Ireland. He spent three and a half years with Drogheda United's U21 squad before moving to Drogheda Town at the start of 2008. In June 2008 the non-league side were drawn against Bohemians in the third round of the FAI Cup. Bohemians won the game 3-0 but Sean did well enough that one month later, in July, he was snapped up by First Division Monaghan United
Sean's debut for Monaghan came in a 2-0 win over Shelbourne on 29 July 2008 and three days later he scored his first goal in a 2-1 win away to Kildare County. 2009 was a season plagued with injury but Sean regained fitness towards the end of the year and in 2010 he made 27 league appearances and scored eight league goals. Sean further enhanced his growing reputation in 2011 when 26 league appearances yielded 13 goals as Monaghan won promotion to the Premier Division.
In December 2011, Brennan moved to Drogheda United where he rejoined his old Monaghan boss Mick Cooke. He made 24 league appearances as the Drogs finished second to Sligo Rovers in the Premier Division, and won an EA Sports Cup winners medal when Drogheda overcame Shamrock Rovers 3-1 in the final.
In November 2012 Sean signed for Shelbourne and he went on to play in 28 league games during the 2013 season as Shelbourne finished bottom of the Premier Division and were relegated. Sean scored two goals which helped Shelbourne to wins against Bohemians (3-0) and UCD (2-1).
In November 2013, Sean signed for his former manager Mick Cooke at newly promoted Athlone Town and he played in 31 of the club's thirty-three Premier Division games, scoring twice. But a dreadful start to the year sealed Athlone's fate and they were relegated at the end of the 2014 season. In January of 2015 Drogheda United announced that they had signed Sean for the third time, reuniting him with another former manager, Johnny McDonnell.
Sean went on to make 26 league appearances for Drogheda in 2015, scoring once in a 4-2 win over Bray Wanderers, as Drogheda United were relegated on the last day of the season. Sean remained at Drogheda United in 2016, under new manager Pete Mahon, and scored five times in 23 league appearances. He also scored in the play-off final second leg as the Drogs won back their Premier Division place with a 3-2 aggregate win over Wexford Youths in the final.
Sean then re-signed with Drogheda ahead of the 2018 season. Sean then re-signed with the Drogs for the 2019 season. Sean re-signed with the Drogs for 2020 shortly after the 2019 season ended. In August 2020, Drogheda announced that Sean was retiring with immediate effect. He made 178 competitive appearances for his local side, scoring 34 goals. That included two goals in four appearances in 2020, against Cobh Ramblers and UCD.