I found a mailing list post saying that IronRuby doesn’t build with xbuild (mono’s implementation of msbuild). So, I decided to give it a shot with latest xbuild. I tried the git repo git://github.com/ironruby/ironruby.git with mono 2.4.2.1, and after a few minor filename casing changes, it builds successfully !
jredville confirmed that the built binaries pass their language test suite 
The patch is being reviewed by cj and jredville at #ironruby/freenode.
To try it yourself, apply the patch, and build as:
$ cd ironruby/Merlin/Main/Languages/Ruby
$ xbuild Ruby.sln /p:TreatWarningsAsErrors=false
The binaries are in “ironruby/Merlin/Main/Bin/Debug”.
$ mono ir.exe
