Major Sanford's Attitude Toward the Wounding of Eliza (Irony)
Major Sanford writes very early on, "I think it would hurt my conscience to wound her mind or reputation" (23); this is precisely what happens, and to a grave extent. This is ironic because Major Sanford is practically the sole cause of her ruin.
Eliza's Attitude Toward Infidelity (Irony)
"I despise the married man or woman, who harbors an inclination to partake of separate pleasures" (61), Eliza writes, but she still loves Major Sanford when he abandons his own wife in pursuit of her.
Blaming Eliza (Irony)
Foster's whole novel can be perceived as ironic, especially when Eliza is blamed more and more for her own downfall when it is clear that her society and the Major played large roles in it.
Irony
It is ironic that Major Sanford, the seducer and avowed hater-of-marriage and lover-of-fortune, comes to love Eliza, wish he had married her, and end up penniless and guilt-ridden.