To be frank, the src/ folder is the compiler for Mirth written in Mirth, so there's your example code. However, I do agree that a quick introduction would not be out of place.
I want to see a procedure call, "Hello world" I/O, some math or string manipulation, and how to run that myself.
After that I can look at compiler sources.
Doesn't look simple to me.
This one is in one file, but more than half a megabyte of code spanning close to 12000 lines:
https://github.com/clisp-lang/clisp/blob/master/src/compiler...
The Emacs Lisp byte compiler also doesn't look like a good small example for neophytes:
http://git.savannah.gnu.org/cgit/emacs.git/tree/lisp/emacs-l...
But even so, I know I'd like a good reference of what primitive operations are, the type system, and what's provided by the Prelude.