Intro
If you've read even one page of Ulysses, it's easy to see why Joyce's text would appeal to Derrida. It's all about play at every level. Words spin out of control, and their sounds become more important than what they mean.
In Ulysses, language itself—in all its over-the-top wildness and weirdness—is the focus. Reference gets left in the dust. In other, Derridean words: there really is no outside-text in this text.
Quote
Ineluctable modality of the visible: at least that if no more, thought through my eyes. Signatures of all things I am here to read, seaspawn and seawrack, the nearing tide, that rusty boot. Snotgreen, bluesilver, rust: colored signs.
Analysis
These sentences open the "Proteus" episode (Episode 3), which centers on Stephen Dedalus. Stephen's just come to the beach. While there, he reflects on his surroundings. But as is so often true in Joyce's works, it's the way Stephen does this reflecting more important. Not what the dude is thinking about.
Notice that so much of what Stephen sees "through [his] eyes" is described as textual rather than simply "visible": "Signatures of all things I am here to read"; "colored signs." Then notice how Stephen (whose stream of consciousness Joyce is very, very close to here) gets carried away by the words that come to mind, like "seaspawn and seawrack" and so on.
Derrida and his students would have a field day with this passage. So perhaps, just perhaps, our boy Stephen was a deconstructionist at heart.