He was signed by John Gill for Dublin City, however, less than a year later, newly appointed boss Roddy Collins' 'purge' of the club meant that Brennan was deemed surplus to requirements. Immediately snapped up by Stephen Kenny for Derry City for the 2004 season, the player was a revelation and made the left wing spot at The Brandywell his own after having made his debut in a home game against St. Patrick's Athletic on 23 September 2004. He has also been deployed to fill in at left full-back on occasion. In the 2006 season he played in their UEFA Cup run in wins over IFK Göteborg and Gretna FC and helped Derry to the FAI Cup and League of Ireland Cup.