Hi Dave,
You're right, the husband would be entitled to 1/3 on an intestacy, but worse (from your perspective) even if a will is made, the husband would have a right to elect to take 1/3 of the estate under Section 111 of the Succession Act, 1965, so if he was left nothing he can still claim 1/3. Accordingly if he exercises that right, then his share would go to his estate on his death, and presumably to his children.
If you mother doesn't want this to happen, it would be prudent to transfer the property to you and your brother subject to a right of residence in favour of herself and her new husband prior to the marriage. It's a relatively straightforward transfer, and any competent solicitor could handle it. It would be no harm to get the husband to endorse his consent on the deed as it prevents misunderstanding in the future. If done after the marriage, his consent WILL be required as it will protected under the Family Home Protection Act, 1976
As to costs, the only direct cost would be the legal fees, and Stamp Duty (check with Revenue), but in the longer term, you have to remember that you will be the owner of the property and responsible for maintenance, insuring it etc (in fact your mother may not be able to insure it).