Keane's Celtic career began in January 2006, when the Glasgow giants crashed to a 2–1 defeat to Scottish First Division side Clyde in the third round of the Scottish Cup. His abrasive style had not dwindled, as he was seen criticising some of his new team-mates during the match. Keane scored what turned out to be his only Celtic goal a month later, a shot from 20 yards in a 2–1 Scottish Premier League victory over Falkirk. He retained his place the following Sunday in his first Old Firm derby against Rangers, leading Celtic to victory. Celtic went on to complete a double of the Scottish Premier League title and Scottish League Cup, his last honour as a player.