Well, Arnold, I found this version of Christy Moore singing "Night Visit" on YouTube:

https://www.youtube.com/watch?v=LpyLGjsRM9Y

I turned on my music program, played the video, then played notes on my piano keyboard till they matched what Christy was singing.

It doesn't have a harp intro, but he's singing in the key of A and the first note is an F#. I think it's modal. You play the note B a lot for a song in A.

Chances are the key was selected to suit his voice and he sings it in the same key all the time.