My understanding:
Whether the ex has contributed or not, he's legally part owner of an Irish property.
He'll need to complete a capital gains tax return for Revenue, in order to get tax clearance certificate (even if there's no gain).
I think there's another option getting a letter of 'no audit' from Revenue, but might not serve the same purpose in this instance - that's really to cover the solicitor.
Your solicitor should ask the law society for guidance. While rare, your circumstances aren't by any means unique.