Got it working with MacOS 11.2.3 (Big Sur)!
I was able to download the newest Adobe AIR from here: Harman AIR Runtime (from Adobe's blog post regarding Harman taking over AIR development). Download the AIR runtime. Install from the .dmg file.
I then downloaded the Bosca Ceoil V2 Windows or Mac AIR script (boscaceoil_v2_windows_or_mac.air) linked above. I installed the script via AIR.
The first time I ran Bosca Ceoil the GUI looked crazy, lot's of horizontal black lines all over. Quitting and re-running the app got it working fine. Yay!