Folan is Boro's second signing this week, following the capture of Sean St Ledger from Preston North End on Tuesday.
He has signed a three-month loan deal with the club, and is expected to complete a permanent switch for £3million int he January transfer window.
Danny Shittu, a Nigeria international, has turned down a loan move to Sheffield United.
He dropped out of Gary Megson's first-team plans at Bolton Wanderers since the summer signing of Zat Knight from Aston Villa this summer.
Shittu, 29, is believed to be hoping to seal a switch to another Premier League club in the January transfer window.