19-year-old Deeney is a Northern Ireland youth international and has been linked with several big clubs including Blackburn Rovers.
Tigers boss Peter Taylor said: "We're delighted to have picked up Deeney. He's played for the Republic of Ireland U-19s and is an outstanding prospect.
"We would love to sign him but you've got to be realistic. The lad is already being watched by several Premiership clubs and he'd probably be out of our reach."
He added: "He's very happy to be here and very happy to be our number two"