Celerity with Watir

I’m trying to use celerity in my Ruby scripts.
I have installed JRuby and Ruby but when I run a Ruby script that
includes Celerity I get this error:
Celerity only works on JRuby at the moment.

As it says on celerity web page it is possible to use celerity with
watir then I don’t understand why I get this message.

Any idea?

You need to call your Ruby script with JRuby.

If you have both Ruby and JRuby installed you need to invoke jruby
By default the ruby executable is in your system path, but it is not
in most jruby releases.

Alternatively the RVM (Ruby Version Manager) receives strong praise for
handling these issues quite well.

but the thing is I need to use certain external ruby libraries at the
same time so I cannot call it using jruby