I´m pretty sure he came sooner than 2006, he played some pre-season games in the summer of '06 and I find it hard to believe he would have been played in those had he just arrived at the club. According to Wikipedia he came to United in 2004.
Also, he wouldn´t have come from Greenisland FC, who, according to their official website, have only boys up to the age of 16. It would actually make sense if he came to United from there in 2004, when he was 16.
edit: it says in his profile on Manutd.com that he came to United in July 2004 and that he actually traveled a few times with the first team in the 05/06 season.