Why did Father Gilligan ask God to forgive him in the poem "The Ballad of Father Gilligan?"

Here's the first four stanzas of "The Ballad of Father
Gilligan," by Yeats.  They include the details you refer
to:

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 have no rest, nor joy, nor
peace,
For people die and die’;
And after cried he,
‘God forgive!
My body spake, not I!’

He knelt, and leaning
on the chair
He prayed and fell asleep;
And the moth-hour went from
the fields,
And stars began to peep.

I've
emboldened the lines that include what the priest sees as his grievous sin.  Actually,
his reaction is probably quite natural.  Members of his flock are dying faster than he
can deliver the Last Rites to them.  He is dozing off in his chair when he gets summoned
by yet still another dying parishioner.  He is overworked and exhausted, but when he
reacts to the summons in a very human way, he sees his words and behavior as not fitting
for a priest.

God, in the poem, though, seems to
understand.  While the priest is praying, begging for forgiveness, he falls asleep and
misses the man's dying moments, but God sends an angel in the priest's place to
administer the Last Rites.