The Scottish giants made a last ditch bid to sign the Northern Ireland international, when it looked all the world that he was going to make a move to Premiership newcomers West Bromwich Albion.
The talented winger, who is widely regarded as the most skilful player in Ireland, held talks with the Baggies on Wednesday, before Gordon Strachan's men made their move.
For the 24-year-old he admits that it's a dream come true and that the last 24 hours has been somewhat of a world wind.
"I'm thrilled and it still hasn't quite sunk in yet to be honest this is a dream come true for me," said McCourt.
"There was interest from a few clubs but as soon as I found out that Celtic were in for me, my mind was made up. I have always been a Celtic supporter and there was only one choice to make. It's an honour to join one of the biggest clubs in the world.
"It's been a whirlwind 24 hours, but I'm just delighted to get everything sorted out. To sign for this club is a dream and when I get back home tonight, it might all finally sink in. It's just been an incredible day.
"I'm joining one of the biggest clubs in the world. Celtic have won the SPL title three times in a row and compete in the Champions League every season. What more could you ask for?
"My first target is to get a good pre-season under my belt and then take it from there. The quality in the Celtic squad is unbelievable but I'll be trying to push my way into the manager's plans as soon as possible. I can't wait to get started."
For Derry the deal could net them in the region of £400,000, with two glamour friendlies lined up against the Hoops.
"As part of Paddy's move we have agreed to host two games against Celtic," said chairman Pat McDaid.
"The first match is to take place on Tuesday, October 7, with the second game scheduled as part of Celtic's pre-season campaign next summer."