This is another instance where Daniel Cassidy got it massively wrong in his absurd insult to the world of scholarship, How The Irish Invented Slang.
There is a common word in Scots (the Lowland Scots version of English, not Scots Gaelic) which is also found in northern dialects of English, the word gallus. According to the experts, this word is related to the standard English ‘gallows’. It is used in the plural as galluses in Scotland and northern England as the equivalent of ‘braces’ in standard British English or ‘suspenders’ in American English. The word gallus is also used in Scotland as an adjective which originally meant ‘pertaining to the gallows’ (a bit like the English ‘a gallows bird’, a criminal), but which later meant ‘daring’ or ‘cheeky’ or ‘impressive’. It is still very much alive in Scottish speech.
The word gealaisí in Irish means ‘braces’/’suspenders’ and is a borrowing of galluses. We know that it came from Britain to Ireland rather than the other way round because galluses is found in both Scotland and England and has a recognised etymology (galluses=gallows) while gealaisí doesn’t. Cassidy tries to suggest a tenuous link with the Irish gealas meaning ‘brightness’ or ‘ray’ but fails to explain how this could become an adjective or how the meanings of gealas could give rise to gallus or galluses, or indeed how you can explain an Irish word becoming so widespread in northern England.
In other words, Cassidy’s item on gallus is just more anti-intellectual garbage from a fool who couldn’t be bothered doing any proper research himself but liked to taunt and insult genuine scholars in Ireland, Scotland, England and the US while borrowing extensively from their work when it suited him and selectively ignoring anything they wrote when it didn’t.