Ochtó bróg means “eighty shoes”, so forty pairs of shoes. Although leathbhróg (literally “half-shoe”) means “one shoe of a pair”, it’s péire bróg that’s used for “a pair of shoes”.
I wonder if compound words like leathbhróg, which refer to one of a matched pair of things, derive their second half from a genitive plural rather than from a nominative singular? That is, would the literal translation of leathbhróg be “half of shoes” rather than “half-shoe”?
One option is : Cúig lá is trí chéad seasca sa bhliain. (Found on Quizlet : https://quizlet.com/121469767/set-1-6-flash-cards/) OR Trí chéad seasca cúig lá sa bhliain. (From : http://nimill.blogspot.com/2010/03/365.html?m=1)
I know this answer is coming a long time after your question... :-(