Indeed, Irmin is a library to implement data stores which can be freely cloned, updated and merged along a model similar to git.

It separates the content from the actual store. So it can run as well on a file-system, on a git repository, in memory or in the browser (See for instance https://github.com/talex5/irmin-js).

As git, Irmin uses 3-way merge but applied to arbitrary values and not only files. As described here http://gazagnaire.org/pub/FGM15.pdf and generalized here https://kcsrk.info/papers/oopsla19-mrdt.pdf