Let's take this geomorph I did for my F-series as an example. It's got two rooms or areas that I really should have made more regular, perhaps. The central room is probably the worst to try to describe.
But you don't need to be exact. I would describe it as "an irregular cave that's almost a 30'x30' room with the northeastern corner cut off. Openings in the middle of the east wall, west end of the southern wall, and north end of the western wall."
Similarly, the room in the lower right is "an irregular 30'x20' cave with exits in the south and east." The fact that the northwest corner is more of a diagonal isn't even a big deal, although you could mention this later if the players examine the room in more detail.
It doesn't matter if the players map this absolutely correctly, getting the angles right; all that matters is knowing which directions they can go, where any barriers are, and whether there's any major, suspicious gaps in the map. A lot of irregular rooms can be described as combos of two or more regular rooms. L-shapes are a simple example: "The door to the north opens on a 20'x40' L-shaped room; the L extension is a 20'x20' square area connected to the far end of the eastern wall."
I think it's imperative that we mappers should plan maps with this in mind. When drawing irregular rooms, can you describe them as being roughly rectangular or triangular, or two or three such rooms stuck together? Can you describe the room as a rectangle with an outcropping that juts into the room a couple feet? Can you compare the shape to one or more letters? If yes, go with your design. If you're having a hard time simplifying the description, change the room to make it easier to describe.