IronRuby doesn’t currently work with the latest versions of cucumber. This is a known bug and is a real pain. There is a work around, and it’s kinda rubbish: don’t use any versions of cucumber > 0.6.3. After that version, Gherkin became a .c extension – and IronRuby doesn’t like .c extensions.

Keith Burnell (the .Net Dev Dude) has blogged about getting Cucumber working with IronRuby – you may want to take a look if you’re getting stuck.