"Re: > tallied the keys of the first 60 songs (meaning tunes)
in O'Neill's Music of Ireland."

My reply: I went to high school, and I know the difference between a song and a tune. When I said "song" I meant "song". If you prefer, you can call it a lyric, a ballad, a ditty, an air or a love song.

"Who knows what key these tunes were written in?"

It doesn't matter what key they were written in. Since they were songs, the key depends on the voice of the singer or on the instrument they were played upon.

Some of the pieces could have been composed by people who couldn't tell you what key they were in. The composer had a tune in his head and words to speak, and on different days he might have sung it in different keys.

Others were no doubt written by people with plenty of musical knowledge. It isn't true that everybody in Ireland did everything by ear.

Today, the habit of using D and G is convenient for many people. How could you have jams if people memorized their favorite tunes in a dozen different keys?

Clearly, D and G are also useful for certain instruments - D flute, D whistle, guitar, and fiddle. (When I want my friends to do a tune that's in C, I usually change it to D for them.)