Yes, channel list, user list, lots of white space next to the usernames, for my taste there's also excessive spacing between the single message lines, the channel topic line could take up half of the space, and so on.

I know that many modern apps follow this pattern and it may have to do with making things responsive and work well across many different devices and platforms, but I simply cannot be happy with low information density.

YMMV. It's good that there are so many different clients to chose from. IRC still rocks.