Ballads tell a story in rhyme, and in "The Ballad of Father Gilligan," Yeats's speaker relates the story of how on one particular evening, God showed mercy on both a tired old priest and a man at the moment of his death. The theme is God's omnipresence and grace in the lives of the faithful.
The first stanza implies that Father Gilligan's congregation is dwindling; half are either dead or confined to bed. Father Gilligan is an old man whose religious duties are physically tiring, and he struggles with the limits of his aging body when duty calls in stanza two.
In his infinite mercy, God allows Father Gilligan to fall asleep and rest through the night while his parishioner is dying. He sends an angel to the dying man's house in Father Gilligan's place, and the man's widow identifies it as Father Gilligan when she speaks to him in the tenth stanza.
The last two lines of the poem observe the theme: though God has "planets in His care," he attends to the physical and spiritual needs of all his people—including an exhausted priest, who humbly considers himself "the least of things."