Like Liam, I'm not an accountant, so not qualified to advise. As an interested layman, however, my 10 cents worth. You probably declared your 2015 pension via Form 11 and put the whole of your cash - in sum as taxable overseas pension income. This is what needs to be corrected. From the Irish Revenue tax manual, mistakes arising in tax paid as a result of submission errors any time after 31/12/14 can only be corrected by submitting a revised tax return. You have a 4 year window to achieve this, so should be in time. What I suspect you need to do is submit a revised Form 11 for 2015, this time showing 25% of your cash - in value in the lump sum tax free "box". (And I don't know what "box" this would be). I believe there is an opportunity to add some narrative along with the revised return explaining the reasons.
Good luck.
Paul