Suggested description, based on Facebook image (for reference only!):
(line-by-line version since it's structured as such.)

(inscription in Gaelic, as signed);
"
Curtha suas mar chomóradh ar
Chothrom 50 Bliain
an lae ar tugadh Caisleán Chill Chainnigh ar láimh go foirmiúil
ar an 12ú Lúnasa 1967,
ag Arthur Butler, an 6ú Marcas Urumhan, chuig
Cumann Athchóirithe agus Forbartha Chaisleán Chill Chainnigh Teo.
"

(inscription in English, as signed);
"
Erected to commemorate the
50th Anniversary
of the formal hand-over of Kilkenny Castle,
on the 12th August 1967,
by Arthur Butler, 6th Marquess of Ormonde,
to the Kilkenny Castle Restoration and Development Society Ltd.
"

(bottom right corner):
[ logo of The Office of Public Works (OPW) ]