Richard, if the tune you are using for "Silkie" is the same one that Joan Baez sang on her record, I think you'll find that the A chord you recommend is going to "clank" pretty badly, because on the first change, the note being sung is C, which is not in the A chord (A-C#-E). In the Mixolydian mode in which the melody is set, if you're singing it based on D as the tonic, there is no C# in the key.

You might be able to get away with an Am (A-C-E), but not an A major, especially at the points where you'd be singing a C and playing a C# in the accompaniment!

The otherwise major scale with the flatted 7th is what defines it as Mixolydian mode.

I'm not making this up. It's not just my opinion. Some five years of formal study of music theory and music history.

Don Firth

P. S. And modes are not just a "religious invention."