As we all know Ireland merged with the United Kingdom of Great Britain in 1801 to become the United Kingdom of Great Britain & Ireland. The separate Peerage of Ireland did not disappear and several titles were granted in it throughout the 19th century, subject to certain conditions; the following is taken from Wikipedia:
Irish peerages continued to be created for almost a century after the
Union, although the treaty of Union placed restrictions on their
numbers: three needed to become extinct before a new peerage could be
granted, until there were only one hundred Irish peers (exclusive of
those who held any peerage of Great Britain subsisting at the time of
the union, or of the United Kingdom created since the union)– a
condition still not achieved
I question whether or not the condition has been achieved, and I think it might have been. I count only 68 peers who hold only peerages in the Peerage of Ireland. Obviously it is now inconceivable that the Crown would create new peerages in the Peerage of Ireland, so it is an academic point.
There are likely to be the following movements in this number in the next few decades:
Likely new "only Ireland" peers:
Viscount Powerscourt (when Baron Powerscourt (UK) likely becomes extinct on the death of the current Viscount)
Earl of Clancarty (when Viscount Clancarty / Baron Trench (both UK) likely become extinct on the death of the current Earl) - not sure if the heir to the Earldom found by members of this group is acknowledged by the published sources yet.
Likely losses of "only Ireland" peers:
Earl of Westmeath
Earl of Norbury
Viscount Southwell
Baron ffrench
Baron Clanmorris
One might also add Baron Trimlestown to this list - no heir is known, but the title is so ancient it may not be possible to definitively prove extinction.