Explain "under green sods lay," in W. B. Yeats' poem "The Ballad of Father Gilligan."

A widespread epidemic has devastated the Irish countryside
and the poor village people have succumbed to it and lie buried under the ever green
Irish countryside.

'Sod' means the
layer of earth with vegetation on it. In the first stanza
Yeats describes to us the weariness of Father Gilligan. He tells us that because of the
epidemic almost half the village has died and lies buried "under the green sods" while
the other half is lying in their sick beds. So, Father Gilligan the parish priest is
kept busy day and night because he has to either conduct the funerals of his dying
parishioners or administer communion to the remaining parishioners who are either sick
or on the verge of death.

In the Bible the
Priest is often referred to as a shepherd and his parishioners as his flock.
So, Father Gilligan is equated with a shepherd who tends his parishioners
who are his flock.

Ireland is an island and enjoys rain
throughout the year and consequently the Irish countryside is always green. The color
green is also the national color of Ireland and Ireland is picturesquely referred to as
"the emerald isle."

The irony in this line is of course,
that sheep usually graze on the green grass but here because of the devastating epidemic
which has caused an ironic reversal, the sheep are lying buried under the "green
sods."