James Joyce wrote in English, no Irish person pretends that he wrote in a third language distinct from English and Irish. The fact that they do not do so does not compromise the political basis for independence, republicanism or reunification.
If a Cork taxi driver, addressed you in Irish (very unlikely), and you asked him to speak English, the request would be both coherent and reasonable. The point you missed is that the Glasgow taxi driver would look at you with consternation and say "But, I am speaking English! What's wrong with my English?' (insert dialect spelling if you like)
Rabbie Burns wrote in the same language as his compatriots Louis Stevenson and Scott.
It would be ignorant if I did not know about the meretricious claim of a minority of Scottish people to have their own language, but it is not ignorant to reject that claim. I am Scottish fwiw.