What do we mean by "dark" and "ironic"? Let's just give you an example, straight away.
At one point in the novel, we hear the story of an executive at an electronics and rockets company who is replaced by an IBM 7094. His wife immediately leaves him, and, after several weeks of deliberating, he decides to immolate himself like the Buddhist monks who protested the Vietnam War. But after he covers himself in gasoline and is about to light the match, his wife comes home with her new lover: the efficiency expert who fired the executive. The efficiency expert sees what he is doing, and says:
"Nearly three weeks it takes him to decide. You know how long it would've taken the IBM 7094? Twelve micro-seconds." (5.71)
That's halfway between "Hey-o!" and "Ugh, that's messed up." And that's pretty much the tone of the entire book.
This novel is full of sick, hilarious jokes. In other words, it's classic Pynchon. He seems to view all of his subject matter through a darkly ironic mirror, and often sacrifices realism for absurdist comedy or plots so complex that they can only be born out of paranoia. We don't sense that the narrator has much sympathy for any of the characters (with the possible exception of the Maas's). In fact, Pynchon hardly even takes the time to give most characters an identity beyond an absurd name like Genghis Cohen.
Some people have major problems with Pynchon for this very reason. At times, it feels like Pynchon is just messing with the reader for the fun of it. After all, there aren't too many books that you finish and wonder if they're a profound meditation on the nature of America or just a grand practical joke.