This is taken from Nótaí Beathaisnéise in Scéilín ó Bhéilín -

Roibeard Ó Cathasaigh: Is ó Lios Póil i nDuibhneacha do Roibeard......

I don't quite understand the construction around the word Duibhneacha and what 'do' is doing here.
Is the word Duibhneacha being used to say where Lios Póil is or is it an adjective describing Roibeard Ó Cathasaigh ie that he's a Corca Dhuibhne man?

There are two (four?) ways to say 'R is from LP': 'Is as/ó LP (é) R' or 'Is as/ó LP do R'.

So your sentence means 'R is from L P in D'.

OK so if you can say - Is ó Lios Póil do Roibeard Ó Cathasaigh - is that an idiomatic use of 'do' ie 'to RÓC' as well as saying 'ó' ? bearing in mind of course that this is in Munster Irish. I looked up the seven different uses of 'do' in Teanglann as well as in the dictionary compiled by David Webb on Munster Irish wordpress and found myself even more confused than when I started.

Yes, it's an idiomatic use of do (cf use no. 6 in 'teanglann'). Another example (not listed there) which is hard to understand: Is ann do + noun/pronoun = Tá noun/pronoun ann = 'exist'. So: Is ann do Dhia/Tá Dia ann - 'God exists'.

'I'm from Lios Póil' - Is ó Lios Póil mé/dom
'X+Y are from Lios Póil' - Is ó Lios Póil (iad) X+Y/do X+Y

Both the mé/dom structures are used in all the Gaeltachts as far as I know, though I think ó instead of as may be more of a Munster thing.

What do you mean by "...as well as saying 'ó'". You need ó or as to indicate 'from' whichever type of structure - mé or dom - you're using.

Because both 'ó' and 'do' are being used in the same expression, which is why I was wondering what the use of 'do' here could add to it and of course I thought that 'do' could only be referring to the person rather than to the place.

You caught me while I was adding a final sentence to my post. Anyway, is everything clear now?

Very much not "another silly question".

Go raibh maith agat