History has shown us over and over again that a society cannot be suppressed if their customs and language are allowed to flourish. It’s a lesson exemplified by the colonies’ treatment of black slaves, and of Elizabeth I’s conquest of Ireland.
One of the first things Elizabeth did was to order was the elimination of the Gaelic language. Since their native tongue was forbidden, parents and priests would teach the children surreptitiously; the lessons could not be written down for fear of discovery. Consequently, words came to be spelled phonetically with only small, isolated populations passing down the original written word. Irish Gaelic survived in these remote nooks of the island, and gained a resurgence after the Independence.
Since the common spelling through the late 19th century was phonetical, you will most often see that spelling in Irish songs of the time. For example, the term “a ghra” (meaning sweetheart) would have been pronounced and spelled “arrah”. For historical accuracy, I use the phonetical spelling in my manuscripts as well.