I had never heard or read the term moth hour before, but am reading the American author Jan Karon's book "In the Company of Others" and she uses it several times. The book is set in Ireland, and there are entries from an old diary scattered throughout the story. Here is one excerpt:
I remember how she slit the throat of a Hedgehog which she commanded me to capture & bring to her in a sack--the creature was rolling in fat for the spring & summer had been especially wet & lush and I had often seen him outside his Burrow at the moth hour surveying the land.
I searched and found that William Butler Yeats (1865-1939) used the phrase in his poem The Ballad of Father Gilligan Here are the first two stanzas:
The old priest, Peter Gilligan,
Was weary night and day;
For half his flock were in their beds,
Or under green sods lay.
Once, while he nodded on a chair,
At the moth-hour of eve,
Another poor man sent for him,
And he began to grieve. ...
I can see from this NGram that the height of the use of the term may have coincided with the publication of Yeats' poem. But would he have coined it, or was it in general use and he borrowed it?
I also noticed that there is a Moth Radio Hour, sometimes known as the Moth Hour, which is all about the art and craft of story-telling . See "Why The Moth?" But it's not really related to the term I'm interested in.
So, after all this, I presume that moth hour is an Irish and maybe British expression. Did Yeats coin it? Is it well known and is it still used? (Do any of you Americans use it?) I also presume it means the hours around dusk, when the moths come out. Is that so?