Older book about princess who fights dragons?

I can't remember the name of this book I read a long time ago. It was set in midevil times where this kingdoms princess who had really long red hair and she fights dragons and cuts her cousins eyelashes off and makes some kind of goo that protects her from fire. Also she's in love with her cousin. I think. Not sure but I think her characters name was Erin or something close to it. And the tittle might have the word crest or crown in it. It was one of those "thou" "thy' king James bible type. I read it a looooong time ago. Please help?