The version of this song which is on YouTube and is performed by Altan is in the key of D, but the first and last chords are A's. That puts it in the Mixolydian mode. That's quaint, but it doesn't change anything about performing it.

Another thing about it is that it's pretty low. Its lowest note is the A below middle C. If a soprano or tenor wanted to sing it, it would be a good idea to transpose it to a new key. The chord transposing wheel, which you find on the net somewhere, would come in handy here.

When I sing music, I like to think about the 'pipe and drum' idea. This is the idea that we can add variety by making some phrases flute-like and others percussive. This song seems to beg us to make the first 'o the wind and rain' flute-like and flowing. The phrase 'o the dreadful wind and rain' is a great one to make percussive. Especially the thick, thudding word 'dreadful.'

It's another way to pass a long, clammy night.