Something that's ironical is wryly funny, especially because it doesn't match up with your expectations. It would be ironical to name your enormous Great Dane "Tiny."
You can describe this kind of humor, situation, or literary device as either ironical or ironic — in the US it's more common to use the latter. A famously ironical passage from "The Rime of the Ancient Mariner" goes like this: "Water, water, everywhere, Nor any drop to drink.” In other words, it's ironical that despite being surrounded by water, the mariner is thirsty. The word comes from the Greek eironikos, "putting on a feigned ignorance."