What is ironic about Nathaniel Hawthorne's allusion to the Madonna and Child in The Scarlet Letter?

Indeed, it is ironic that Hawthorne should propose the
image of the Virgin Mother with her Child; and, yet, its irony is also directed at the
Puritans as Hawthorne points to the spiritual beauty of Hester in spite of her sin of
adultery.  Here Hawthorne ironically criticizes the sanctimonious hypocrisy of the
Puritans that allows the sinner no redeeming qualities, no forgiveness.  There in plain
garb, scorned as she stands upon the ignominous scaffold,is the profound emanation
of Hester's inner beauty of soul and beautiful love for her child, her image of "Divine
Maternity":

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Here, there was the taint of deepest sin in the
most sacred quality of human life, working such effect, that the world was
only the darker for this woman's beauty, and the more lost for the infant that she had
borne.

Hawthorne's veritable
comparison of Hester to the Madonna proves itself later in the narrative as she consoles
the sick and aids the aged, so much so that the scarlet A is interpreted as meaning
"Able" and even "Angel."