Bonny Portmore was originally written in English, but the tune has been used for plenty of other songs; the ones I know of are also in English, but I don't doubt that there are Gaelic songs that use it as well. "Leeder" has already said that his friend has told him that none of those is a "counterpart" of Portmore, so that isn't an issue.

Portmore was not a castle but a mansion house; I have no idea where Lord Conway was born, but there would be no inconsistency in a Gaelic song in praise of such. David is, I suspect, making anachronistic assumptions.

The most useful thread on Portmore is  Bonny Portmore: historical background (it also contains a link to another which should also be read); be aware that, at the time of writing, the messages will appear out of order, so you need to look at the dates in order to follow the discussion.

For some other songs using a form of the same tune, see Info on Pretty Saro?.

None of that is any help with Irish Gaelic songs that use the tune, I'm afraid, but there are people here who should be able to help with that. Meanwhile, did your "Irish scholar" tell you what any of those songs were called? It would help if we knew.