Here are paragraphs six and seven from "The Ballad of Father Gilligan," by Yeats:

Upon the time of sparrow-chirp
When the moths came once more.
The old priest Peter Gilligan
Stood upright on the floor.

‘Mavrone, mavrone! the man has died
While I slept on the chair’;
He roused his horse out of its sleep,
And rode with little care.

The moths in Ireland come again in the morning, and the sparrows chirp.  Father Gilligan wakes up, startled that he has slept through the night, and jumps up.  He yells that he has missed the man's death, prepares his horse, and rides as fast as he can to the home of the man that sent for him the previous night, before he fell asleep while praying. 

Of course, the Father arrives at the house, and discovers that God, in his mercy, has sent an angel in his place to administer the Last Rites to the dying man. 

In this poem before the stanzas you cite, Father Gilligan has been called to the bedside of a man who is dying.

But the priest is very tired.  He kneels to pray and falls asleep right there on his knees, flopped on a chair.  He sleeps all night.

In the 6th stanza he wakes up and is horrified.  He realizes that as he has slept, the man must have died already.

So he gets his horse and rides recklessly to the home of the dying man.  When he gets there, the man's wife opens the door and asks why he is there again.

So at that point we know something strange is going on because of course he had not been there yet.