There are three possiblities:
1. Your existing mortgage is for present and future advances, and is adequate security for the top-up. Legal fee might be €50 plus V.A.T. (i.e. singel consultation, and you take away papers with you to bring to bank)
2. Your existing mortgage is for present and future advances but needs extra stamp duty put on it, and to have this noted in the land registry. some lenders take care of this in house; some make you get it done via your solicitor; circa €200 all in.
3. Your mortgage is not for present and future advances (or, as a matter of policy, your lender insists on a fresh mortgage being signed, stamped and registered). This could cost circa €635 plus V.A.T. plus outlay - total costs could exceed €1,000.00. If your lender is referring to it as a "top-up" a new mortgage is unlikely to be required, so it will be either of 1 or 2 above