I was wondering about this.
If I have just fixed for 7 years at a good rate and my mortgage is switched to permanent tsb, then it probably doesn't matter that much that my mortgage has gone to ptsb. I will be vulnerable to ptsb's exploitation of existing customers after 7 years, but they might have changed by then.
If I have fixed for 2 years, by the time my mortgage is sold, it probably won't matter.
Variable rate mortgage holders are more vulnerable as they might go to a lender they don't like and will have to pay €1,500 or so to switch.
Brendan