The 20-year-old playmaker earned himself a three-and-a-half year deal after impressing while on trial with the Hoops.
The former PSV player marked Celtic's second piece of transfer business in as many days after the Clydesdale Bank Premier League leaders announced the signing of defender Jaroslaw Fojut on a pre-contract agreement on Tuesday.
Speaking of the capture of Ibrahim, whose work permit application will be heard next week, Lennon told the club's official website, www.celticfc.net: "We have been very impressed by Rabiu.
"In my view he is a highly talented and exciting young player and someone who will be a tremendous addition to our squad.
"He is a playmaker, well balanced and a very intelligent footballer.
"We are really looking forward to him joining us."
While Lennon's squad was further replenished, one man nobody at Parkhead wants to see leave the club is star striker Gary Hooper.
His strike partner Anthony Stokes became the latest in a growing list of Celtic players urging Celtic to keep hold of the former Scunthorpe striker.
Southampton have made at least two bids for the 23-year-old Englishman with a reported third of #6million million and could test Celtic's resolve again before the January transfer window closes.
Stokes admits it would be "massive" if Hooper stayed to help wrest the Clydesdale Bank Premier League title back from Rangers.
The Irishman said: "You look at how well he has done since he came here. "He has been outstanding and I hope he stays.
"He scored at Scunthorpe, he has come here and scored goals for fun and I think he would score goals at any level.
"He's a top class player and I play reasonably well when we play together so fingers crossed that we can keep him.
"I can't talk about what the club are going to do but from a personal point of view I would like to think they will do everything they can to hold onto him and I'm sure they will."