On their web site FAQ article 8 Although hair straighteners arent in themselves named, blunt instruments are? If you get a member of staff who wants to be awkward. But if your straigteners are the cordless gas type they would be refused in hand luggage
I've never travelled with Ryanair but I have taken GHD straightners in my hand luggage while travelling with other airlines with no problems. As long as there is no gas or liquid I don't think there should be a problem