"I'm very happy with the deal. I'm leaving one great club for another great club," he said.
"There were a few others interested but as soon as I heard of Celtic's interest and met Martin O'Neill that was it."
"I spoke with Rangers and some clubs in the Premiership and said I wanted to have a good think about where I should go.
"As for Rangers I just spoke over the phone with the manager. I didn't speak with the chairman.
"After Liverpool playing here in European competition, I know all about the atmosphere that is created by the Celtic support here in the stadium and I can't wait to experience that in a Celtic shirt," he added.
"I am also looking forward to meeting up with the players here after those Uefa Cup games and seeing so much of Celtic on TV in the Champions League.
"There are 15 games left in the league. I hope to play a part in not only retaining the title for Celtic, but also adding the Scottish Cup to lift the double."