Is that actually an IBA recommendation or the journalist's lazy interpretation?
All I see is a very specific case where someone is on a tracker of ECB +1% would pay 25% less than someone on the current PTSB standard rate of 5.2% over a 20 year annuity mortgage.
It's quite a generalisation to say a 25% writedown is appropriate for everyone with a tracker.
As an aside, here is the calc:
(1 - (1.02 ^ -20))/0.02 = 16.35
(1 - (1.052 ^ -20))/0.052 = 12.25
12.25 is 25% less than 16.35
I see the IBA have called it a "study", poor fools probably paid an actuary a few grand for that...