Jackett knows the 18-year-old from his time as a reserve team coach at Eastlands.
The Dublin-born teenager joins former City team-mates Marc Laird and Ashley Grimes at the New Den.
Meanwhile, Ryan Smith has returned to the Lions after failing to win a permanent contract with NEC Nijmegen.
The 21-year-old winger had been on trial with the Dutch club earlier this month, but a statement on NEC's website read: "NEC has decided the two players who last week trained with the club will not be offered a contract.
"After the trial period of Pedrag Pavlovic and Ryan Smith, technical staff are not convinced of the additional value of both players against the players from the current squad."