Brennan began his footballing career at Sheffield Wednesday F.C. but despite impressing at youth and reserve levels, he never made a first team appearance for the Owls. After being released by Wednesday in the summer of 2000, Dean returned home and was on trial at Bohemians. He made 2 appearances for the club in the FAI Super Cup but Roddy Collins decided not to sign the midfielder.

Dean then went on trial at Luton Town and this time it was more successful, earning a 2-year contract at Kenilworth Road. He made his debut for the club in a 2-1 defeat at Wigan Athletic soon after. Later in 2002 Dean was signed by Hitchin Town.

Deans other clubs include spells at Stevenage, Grays Athletic, Lewes, AFC Wimbledon, Cambridge City, Chesham United and of course Hemel Hempstead Town. At Wimbledon he was nicknamed "the Irish Beckham", a reference to his crossing ability.

In August 2009 Dean signed for Spartan South Midlands Football League Premier Division side Aylesbury FC.