If the money is lodged in to the mortgage account and it sits there as a credit you will only pay interest on the balance outstanding as interest is calculated on a daily basis. I am afraid Clubman is 100% wrong on this as he has been in the past.Your daily capital is reduced by the amount of money that you lodge and you pay interest on the balance outstanding. However your capital loan is not reduced until such time as you notify the financial institution that you wish to capitalise the credit in your account.
It is possible to have a loan of say €100k with a credit of €x over and above your monthly repayment, made up of the extra monies that you lodge monthly, this will show up as a credit on your account because you have lodged more than your monthly repayment. This credit amount can be carried forward for as long as you wish. If you wish to reduce the capital amount on your mortgage you ask the financial institution to capitalise this credit. This in effect clears the credit on your mortgage that you have built up and further reduces your monthly reducing €100k loan by the lump sum amount of the credit.