You won't be reducing the principal, so you'll just be renting €167k from Ulster Bank for a year.
At 5.4% your interest bill for the year would be €9,018 or €751 a month.
When you go back to paying principal and interest, you will either have to make higher monthly repayments to repay the mortgage within the original term, or ask your bank to extend the loan term by the same period as the interest only period and keep your original payment.
They might also charge you an arrangement fee, you'd have to ask them. If you are on a fixed rate or tracker mortgage, then going interest only might not be an option.