 In June 2004 he was appointed to his first managerial post, taking charge of Dundalk, who were also the first club in Gannon's professional playing career. The side finished sixth in the League of Ireland First Division in the 2004 season. On 14 November 2005, Gannon confirmed his resignation from Dundalk after his position had been called into question as his chief supporters, vice-chairman Tom Baldwin and chief executive Sean Connolly, left the club. This left Dundalk without a manager for their final match of the 2005 season, in which they had already secured a comfortable mid-table position in Ireland's second tier of association football. Prior to Gannon's departure, the team had drawn ten consecutive matches.
