The 33-year-old, who was the mainstay of the Bees midfield under Mark Warburton as they reached the play-offs last season, is currently undergoing a medical at Portman Road.
Sky sources understand Douglas has agreed terms on a one-year deal with the option of a further year, after being told he was unwanted by Brentford’s new manager Marinus Dijkhuizen.
Douglas, who has been capped eight times by the Republic of Ireland, had signed a new Bees contract until 2016 only last February.
Douglas began his career as a Blackburn trainee and made 14 league starts for the Lancashire club amid loan spells at Chesterfield, Blackpool, Gillingham and Leeds.
He signed permanently in 2006 and made over 100 league appearances for them before a move to Swindon. Douglas then made 187 appearances and scored 17 goals in four years at Brentford.