There are few pluralia tantum (plural only nouns) in Irish but there are quite a lot of them in place names.
The best known are the names of the provinces Connachta, Ulaidh, Laighin. But there are more

E.g. Cairbrigh, Carbery. The "Cairbrigh" (or Uí Cairbre) were originally a tribe living in that area. But now it is the name of a territory or the name of some territories and baronies (Cairbrigh Thiar, Cairbrigh Thoir agus a rannta).
According to logainm.ie its genitive form is still Chairbreach, i.e. obviously a genitive plural. It's old dative form is Cairbreachaibh, obviously a dative plural.

So, I wonder, if it (and similar place names) is still used and felt as a plural noun and if still plural pronouns should be used to refer to it, e.g.
"Is réigiún i gContae Chorcaí iad? Cairbrigh Thiar."

What a fascinating notion! If only I had any kind of insight to share...

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ЯГОН ТОҶИК НЕСТ ИНҶО???