It could have influenced it at least, I suppose, but I can’t think of any case of an English adverb becoming an adjective in Irish.
Knowing when the word entered Irish would be useful. McBain (under “silpidh“) and the DIL aren’t much help.
According to “The Irish of Erris” it’s a small group of nouns ending in /-É™/ that form the plural with /-u:/. The examples given are: cleite, faithne, gearr-chaile, gloine, leithe, reithe, seithe & sine.
It does seem to be a Connacht feature, I found no mention of it in “Gaeilge Theilinn”