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.