I guess "IR" is "intermediate representation" while "MLIR" [1] ("Multi-Level Intermediate Representation") is an IR from Chris Lattner/LLVM that is designed to be modular and extensible. For the uninitiated, I think [2] shows how the current Julia IR is generated and used in the Julia JIT compilation process[1] https://en.wikipedia.org/wiki/MLIR_(software)
[2] https://docs.julialang.org/en/v1/devdocs/jit/