Casey (52) became the fifth dragon to quit the hit series, following in the footsteps of Bobby Kerr, Niall O'Farrell, Sean Gallagher and Sarah Newman.
Norah's decision comes several months after she landed two high-profile roles in broadcasting -- the Breakfast Show on Newstalk, and the Today show every Friday with Blathnaid Ni Chofaigh.
The businesswoman announced her decision this week, saying it was "the right decision for me at this time".
ShinAwil, the production company behind the series, admitted that losing Norah was a blow.
"Norah was a great asset to the show and will be missed. We will be announcing the new dragon line up over the coming week," a ShinAwil spokesperson said.
Several names are already in the mix.
No doubt Ryanair boss Michael O'Leary would be top of producers' lists -- but it's highly unlikely he will do the programme.
Gerald Kean has also been tipped, but given his reluctance to feature in TV3's series Dublin Wives, Kean may refuse.
Republic Of Telly star Jennifer Maguire -- who previously featured as a contestant in the BBC version of The Apprentice and has her own makeover business -- is in the running for the spot.
ShinAwil boss Larry Bass is remaining tight lipped about the future dragon.
"Nobody knows who is coming onto the show. The dragons have all asked Larry but he is being very secretive," a source close to the production said.
Sean Gallagher left the Den ahead of his unsuccessful run for Aras an Uachtarain last year. Then Black Tie owner Niall O'Farrell and Insomnia boss Bobby Kerr followed suit earlier this summer. The current line-up now has just two dragons -- original Gavin Duffy and American businessman Sean O'Sullivan.
Casey was the second businesswomen to feature on the RTE series. Millionaire businesswoman Sarah Newman, the former partner of Kilkenny hurling great DJ Carey, left the show in 2010.
Norah said that her schedule had simply become too stacked and she needed to make time for herself and her son Darragh (13). Her husband Richard Hannaford (49) died from cancer last year.