There's some dramatic irony to the fact that Richard decides to give Clarissa flowers as a way of expressing his love. What he doesn't know is that Clarissa has already bought flowers herself, but readers do know this. In fact, the very first line of the novel announces Clarissa's intention to buy flowers: "Mrs Dalloway said she would buy the flowers herself." As such, the entire time Richard is walking home and proudly thinking about the grand statement he's making by bringing Clarissa flowers, readers know his gesture won't be all that meaningful. In this way, the narrative subtly pokes fun at him as he excitedly prepares to present the flowers to his wife:
But he wanted to come in holding something. Flowers? Yes, flowers, since he did not trust his taste in gold; [...].
Richard thinks this while standing next to Hugh Whitbread and watching him select jewelry for his own wife. He thinks that by choosing to give Clarissa flowers, he will circumnavigate the fact that he has no idea what kind of jewelry to get her. And yet, buying her flowers only further emphasizes how out of touch he is with his wife. After all, readers know that Clarissa already bought flowers earlier in the day, and though it's perhaps understandable that Richard might not know this, it does suggest that he doesn't have a very good idea of what she does during the day. And this, in turn, only accentuates the "gulf" between them.
Richard brings Clarissa flowers and plans to express his love for her. But when he sees her, he finds himself unable to do so, and Clarissa ends up thinking about how emotionally distant people—including spouses—can be from one another. Richard’s attempt to make a romantic connection with his wife thus only ends up making her feel less connected to him, leading to a perfect example of situational irony. Richard intended to make a gesture that brought him and his wife closer, but the gesture itself ends up making Clarissa feel like there is a "gulf" between them. As he leaves, she thinks:
And there is a dignity in people; a solitude; even between husband and wife a gulf; and that one must respect, thought Clarissa, watching him open the door; [...].
Of course, it's worth noting that Clarissa isn't upset by her interaction with Richard. It's just that she starts thinking about how unknowable people are—everyone, she reflects, has their own private life that is very difficult for others to understand. Although it's ironic that she ends up thinking this after Richard tries to connect with her, it makes perfect sense, considering Richard's complete failure to actually express his feelings. His original plan was to give Clarissa flowers and tell her that he loves her, but he found himself incapable of saying "I love you." Instead, they spoke about ordinary things, and though he leaves feeling as if he has, in some way, conveyed his affection, it's unsurprising that Clarissa feels rather confounded by her husband, since his behavior would surely be quite hard to read in this situation.
And yet, Richard himself seems rather pleased. He thinks he has expressed his love for Clarissa "in so many words," but all he has done is give Clarissa flowers. As such, there's also some dramatic irony at play, as readers understand that Richard's gesture has failed even though he himself is completely unaware of this fact—he thinks he has more or less accomplished what he set out to do. Because readers have access to Clarissa's thoughts, though, they know that this isn't necessarily the case.
