I think she had some deeply essentialist ideas about 'Celts' in general they''re either 'wild Irish girls' like Biddy O'Ryan and Deira O'Hagan, Welsh people exactly as you said, or second-sighted Scots who wander about in tartans. And everyone speaks in a Stage Irish/Welsh/Scottish dialect even Anglo-Irish Deira -- that suggests EBD had never actually met a Welsh, Irish or Scottish person.
Oh, nearly forgot Juliet's useless beau (Dermot?) who 'blarneyed' Joey into helping him win Juliet back after he'd been too wet to stand up to his nasty sister.
On the cookery subject, it was fairly clear to me even when I read the books aged ten or so, that EBD must never have seen a clove of garlic, surely! NO one could confuse cloves with garlic cloves -- did she just assume from the name they must have looked vaguely similar???