You can run irb from vim with the command !irb.
Running !irb -r % should run the current file in irb then leave irb open. Unfortunately ruby 1.9.2 removed the current directory from the load path which causes this to fail. You could construct a path but it is easier to add the current directory back in, especially if you are using irbtools which does it for you.
Once irbtools are installed add:
map <F2> <ESC>:w<CR>:!irb -r %<CR>
to .vimrc.local which will write then load the current file into irb.
Edit: I was getting some weirdness with the irb prompt with irbtools so I have removed it and added
$: << ‘.’ if RUBY_VERSION >= ‘1.9.2’
to my .irbrc file to add the current directory to the load path.