No, it's more expensive but you will pay the mortgage off quicker.
Say you are paying €400 per month at the moment.
€400 x 12 = €4800
If you change to weekly, you will pay €100 per week.
€100 x 52 = €5200
Therefore you will pay an extra €400 off the capital each year.
I think you can pay every two weeks with BOI.
If they do not allow you to pay weekly you can overpay your mortgage instead each month.
Using the above example. €5200 / 12 = €433 per month.
Overpaying by €33 per month will have the same effect as paying weekly.