At the beginning of Chapter 6, Tess reflects on a thorny rose that, affixed to her breast, pricks her and draws blood. She views this as a bad omen, and the narrator takes this moment to foreshadow the tragic events that will soon befall her:
[Tess] fell to reflecting again, and in looking downwards a thorn of the rose remaining in her breast accidentally pricked her chin. Like all the cottagers of Blackmoor Vale, Tess was steeped in fancies and prefigurative superstitions; she thought this an ill-omen—the first she had noticed that day.
There are many literary elements at play in this particular passage. The roses given to Tess by Alec symbolize his harmful, romantic intentions, as he's more likely to "prick" her and draw blood than to bring anything positive into her life. This injury foreshadows the more direct injury Alec will do to Tess later on in the novel, both directly—as he physically harms her when he rapes her—and indirectly, as he damages her chances of finding happiness and love in a married state. It is ironic that in this situation the rose pricking Tess is more of a red flag than Alec's suspicious behavior upon their first meeting. The combination of irony, metaphor, and foreshadowing highlight Tess's innocence and naiveté.
Angel Clare's very name serves as an example of situational irony in Tess of the D'Urbervilles, given that he refuses to enter into the Church as a profession. He also denounces the religious establishment, much to the sorrow of his traditional father:
Once upon a time Angel had been so unlucky as to say to his father, in a moment of irritation, that it might have resulted far better for mankind if Greece had been the source of the religion of modern civilization, and not Palestine; and his father's grief was of that blank description which could not realize that there might lurk a thousandth part of a truth, much less a half truth or whole truth, in such a proposition.
Though it makes sense that a religious father might name his son "Angel," it is a form of situational irony that the character named after a religious figure happens to be one who opposes the constraints of the religion he was raised in. This situational irony accentuates Hardy's critique of the church's hypocrisies, which continues throughout the novel. The conflict between Angel and his father also represents the conflict between Christian values and secular values, the former of which often privileges science and reason over faith in the determination of universal "truths."