The 26-year-old Young joins the English club having been squeezed out of contention for a starting spot in the Ulster side in recent seasons. The rise of Andrew Trimble and Tommy Bowe saw Young backs on the fringes of the Ulster squad, having made his breakthrough under Alan Solomons - 2003/04 was his best season for the men in white when he scored four tries in 20 games.
Doncaster are still to appoint a successor to Clive Griffiths as director of rugby, but can expect to put a good challenge in for promotion next season as they will be operating as "effectively" a full-time outfit, according to the club's Chief Executive James Criddle.
Commenting on the signing of former Irish international Bishop, Criddle said: "Justin will make a difference, no doubt, and he has brought in Scott Young from Ulster who can play centre, wing or full-back. We saw the backs improve significantly later on last season and we are happy with the make-up of the squad (for the coming campaign)."