Assuming its a variable, you can pay whatever you want, whenever you want. Just have the direct debit for the normal amount, related with the 30 years. Then once a month pay your overpayment manually, that will reduce the cost for normal mortgage payment amount for the next month's repayment slightly, so you would slightly increase your overpayment amount, so that the 2 amounts add up to the same each month, say E2000 for simplicity. Keep doing this over and over and you will see the normal monthly going down, and your overpayment going up - I do this now with AIB. If you keep doing this, it will pay off the total earlier, without having officially reduced the term.