Talk to your solicitor. However, if your partner is coming on board, is a first time buyer, and does not mind using up her FTB status, then the logical thing is for your partner to buy your brother's half of the house. This resolves the stamp duty issue.
This transaction does involve a purchase and a new mortgage. For dealing with the purchase and mortgage, the solicitor's fee will be much the same as when you bought the house (and the outlays will be broadly similar too).
You might think it would be maybe a little less as your solicitor has already read through the title documents on the previous purchase, but as against that your solicitor will probably have to draft all the documents for the transaction, including those which would normally be prepared by a vendor's solicitor (certainly this is the way I would deal with such a transaction) so there is an element of swings and roundabouts.
It would be appropriate that your brother be independently advised. On the assumption that your solicitor prepares all the legal papers, your brother's legal costs should be fairly minimal - perhaps €125 plus VAT for one meeting to sign all papers in one go.