a recording and lyrics: https://seannos.tg4.ie/baile/amhranaithe/micheal-o-confhaola/an-casaideach-ban-sean-nos/

I am seeing a few different spellings of the name An Casaideach (TG4, Irish language television spelling) and An Caisideach is used in the Joe Heaney archive website. All observe the spelling rule of pairing slender vowels (e,i) or broad vowels (a,o,u) on either side of a vowel. It's like saying like "the O'Cassidy", as the author's surname was Ó Casaide. I wonder if he leaned much on traditional sources, or if the other versions such as the Donegal rendition of Thug Mé Ruide are evidence of the folk process working on Ó Casaide's composition - or both things happened in the development of this song.