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You need a disguise. Talk to the Wizard Cathbad in Red Branch Knights' Castle. He'll tell you the Green Fairy in Green Fairies' Moor can make you one. You'll have to do a quest for her. Then she'll make you a Connacht Shepherd's Cloak that will fool the guards enough to let you in.
I know this is an old post but can you please tell me how you triggered wizard Cathbad to give you the disguise quest. I have tried getting into the castle via the guard and then speaking to the wizard but he won’t give me the disguise quest for the green fairy. Do I need to do something else first?
Worked it out. Did need to speak to someone else first. Back on track