It really depends on who you talk to in PTSB, you will get a different set of rules from different customer service reps.
I have been given conflicting information on numerous occasions.
you were told it ha d to reach €1500. Well I was first told it had to reach €1000 before you could offset it, then i was told €1270 (??? random figure), then i was told in order to offset against capital i needed to cancel my ongoing overpayment before they would do that.
However i have since learned that the overpaid credit sitting in your mortgage account is taken into account when your interest is being calculated. So it is of benefit there. I overpay by apx €500 per month which i wil leave in credit.
Your mortgage would need to be rescheduled every time you actually offset a sum against the capital, so would involve lots of admin / paperwork etc.
I just wish that PTSB had a clear and concise set of rules regarding their flexible options / overpayments.