Depending on the solicitor it can cost almost as much as the original transaction. First American are a title insurance company that handle all the legal work involved in switching for a flat fee of, I think, €1059.
Most lenders now will cover the costs for clients - NIB, EBS, Bank of Scotland (only if you go direct to them ie not through a broker), IIB (loans greater than €200k). Ulster Bank and First Active also give a cash sum as well as paying legal fees for clients who switch and drawdown before June 30th - it may be worth chancing your arm to see if they will extend this for you.
As bacchus mentioned you will have to get your property valued but this only costs around €130 and some lenders will cover this cost also.
However, that if you redeem your mortgage withing a specified period, usually 5 years, you will have to repay the fees covered by the lender so it's worth keeping that in mind - especially if you're considering selling soon as you mentioned.