The start of this story from the Book of Leinster has the sentence "Ni rucad and do loon for muir acht teora bargin", which Elizabeth Boyle translates "Only three loaves were taken to sea as sustenance."
The Best, Bergin, and O'Brien edition is available on the UCC Cork CELT site.
I thought the word "loon" looks strange, as the combination "oo" is not seen much in Irish.
DIL says that "disyll. loon , loan" was an earlier form of "lón..fat..provisions, food".
The disyllabic examples in the dictionary are the one above from the book of Leinster, and a second example from the Glosses on Priscian. Modern Irish has 'lón' with a similar meaning. I couldn't find another word where disyllabic 'oo' or 'oa' became 'ó'.
I did a DIL search for other words with 'oo' and I found 'óo' and 'oo' as a comparative form of 'óc', also a phrase 'da n-óó' where 'óó' is a dual form of 'ó' ear. On a CELT search, I found a placename 'Ath n-Oo' in 'Echtra Cormac i Tir Tairngri ocus Ceart Claidib Cormaic'. That was all I could find. Liz