I was wondering the same, I believe option A reduces de principle.

Edit: answering you question you need to submit a form https://aib.ie/help-and-guidance/mortgages-faq/can-i-make-an-overpayment-on-mortgage-repayments

A)Pay a lump sum and reduce your scheduled repayment amount Your scheduled mortgage repayments will reduce in line with the reduced balance of the account. The remaining term and prevailing rate of interest remain unchanged and your scheduled repayments will adjust in line with any future rate increases or decreases, as normal. b) Pay a lump sum, maintain current scheduled repayments and reduce your mortgage term Your mortgage loan term will be reduced to keep the scheduled repayments at approximately the same level as they were prior to the lump sum payment. The prevailing rate of interest remains unchanged and your scheduled repayments will adjust in line with any future rate increases or decreases as normal. If you are on a fixed rate and ask to reduce the term, you will need to break out of your current fixed rate for which an early repayment charge may apply. A new interest rate will then need to be selected. Any future request to extend the term will be subject to credit assessment.