Honestly? In theory, it would be very important and it is indeed, but from how you have written your message it does not seem your English is that bad. Even if you have a very heavy accent, it is not a problem as long as you can carry on a conversation and speak without too many breaks in between. At the end it is all up to you and how you feel. More so during the interview. And it might take 2 weeks or 6 months to get a call so try to use English as much as possible in your everyday life, and that means watching movies and series in English and reading if you can't afford to go an English speaking country.
And it's not only important for Ryanair. Just saying...