One instance of irony in Tennyson's "The Lady of Shalott" occurs in the resolution and is situational irony. The Lady of Shalott has seen the "shadow," the reflection, of Sir Lancelot, which compelled her, regardless of her curse, to run to window to see him for herself instead of as a shadow in her mirror.
As soon as she did so, her whole life was forfeit: the weaving flew out the window ("Out flew the web..."), the mirror cracked, the curse had taken it's inexorable hold. The inevitably fatal nature of the hold of the curse, which the Lady felt rather than knew about, is what led her down to the water's to sail in a boat she took the time to name The Lady of Shalott and why she "loosed the chain" (release herself from earthly bonds) and lay down to sing her last and die.
The Lady gave up her life for a glimpse of--probably, since she didn't know the nature of her curse, for a hope of uniting with--the glittering Lancelot. Her boat bore her down the river to Camelot, that haven of goodness and rightness (another irony, also of the situational kind), and to shinning Lancelot.
She dies as the notes of her last song fade from her lips. Her boat floats to Camelot where Lancelot sees her laying there dead. His one comment--the object of her desire unto her death--the one thing he says is, "She has a lovely face." He graciously adds, "May God in his mercy lend her grace," which was a pat and meaningless benediction.
This is situational irony: The shinning triumphant man whom she was compelled to reject everything for is so coldhearted and unfeeling he says nothing more over a beautiful young woman's untimely death than "She has a pretty face." She thought she had seen perfection but in ironic reality she wasted herself on a scoundrel in Camelot.