Hi, I was just painting my bathroom celing and had finished cutting in the edges with a brush and started using my roller. The ceiling was replastered about five years ago so just used special bathroom ceiling paint. Whilst rollering above the shower area the paint came off onto the roller and then in other areas not directly above the shower but just outside that area it started to bubble. I then used a scrapper and a lot of paint peeled off very easily. I have since tried to scrape more off and it has come off easily in some areas but not others. Do I just use a bonding primer on the bare plaster (which looks white to me)?

You say 'whilst,' a word most often used in GB, and suggesting that your ceiling might be calcimine, which hasn't been used much in the US in 50 years or more. What you need to do depends on what you painted over. Calcimine can't be painted over except by more calcimine.
Another possibility is that the bathroom was just too damp. You really can't let it steam up from a shower or any hot water running a while without drying it out for several days (min), with the help of fans or windows open.