Submitted by the author, who says:

Not a transit map, but a cave map inspired by Harry Beck I made a few years ago.

Transit Maps says:

This is fun! This map wears its influence very much on its sleeve, with much of the Tube Map’s celebrated visual language lifted directly from the source, but this is still very nicely put together. The one obvious difference is the substitution of what looks like Avant Garde for Johnston Sans (note the unjoined capital “R” – a dead giveaway). 

From what I’ve read about the Shannon Cave, this seem to be a fairly accurate depiction of the system, complete with landmark names and the different routes of streams.