I think we can be in danger of describing perfectly common ordinary words as slang just because they aren't that commonly used through the entire UK. In particular the word "girn" which is a vey common word in Scotland in both the uses mentioned and is well attested in literature right back through Scott, Burns, Ferguson and right back to Barbour. Basically as long as there has been literature in Scots characters have been 'girnin' ;-)

Not just in Scots it is in the Concise Oxford too. I am sure it is reasonably common in northern England too. Not really a slang word at all.

Re why it is common in Northern Ireland? It may well be from the Irish Gaelic - but it certainly doesn't need to be from that source. A fair percentage of the Northern Ireland population is of Scottish and northern English ancestry. It even has its own recognised dialect of Scots in Ulster Scots. Those people would have known the word well enough without borrowing it from Irish Gaelic.

Maybe of course it came from both sources.