Usually I have a look in my books to search for meanings and origins, sometimes (if I can't find anything) I search the internet. But there are tons of unreliable information there (even in books sometimes but not as often!), so I'm not always sure if the meanings given are right or not...
The information about Eryl I found here:
http://www.cfimages.com/Baby/Names/boyse.html
No idea if that's right or not! ;-)
Regards,
Satu