Boss Lee Bradbury swooped for the Irishman ahead of today's loan transfer window closing at 5pm.
The 21-year-old winger goes straight into Bradbury's squad for Saturday's League One clash with Charlton after joining Cherries until the end of the season.
McDermott, who would also be available for any potential play-off matches, starred on his last appearance at Dean Court.
The Dubliner, who can also play up front, tormented Cherries when he helped Chesterfield claim a 2-1 victory over Eddie Howe's men in October 2009.
McDermott netted five times in 15 appearances during a loan spell with the Spireites and also played nine times in the Championship for Scunthorpe last season.
He joined City as a 15-year-old and helped them win the FA Youth Cup in 2008. McDermott signed a new two-year deal at the City of Manchester Stadium last summer.