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Have searched the text of the Irish Brewers on the Amazon website and both spellings come up, though Jarlath more than Jariath and, looking at the image of the main entry, Jarlath looks more likely. Will just have to accept Jariath though as a possible variant.
Cannot trust Brewers as the final authority any more after discovering that Helen of Troy was the daughter of Thetis (see under Momus) and that the Maid of Kent protested against Henry VIII's divorce from Anne Boleyn (both in last 2 editions of P&F!). Have emailed editors so hope these will be corrected, but it destroys one's faith in them.