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Generally speaking, the 'but' vowel tends to occur in standard English, the kind that sounds as if it isn't being spoken with a regional accent, and the 'bull' vowel is one of the features of regional accents. So, most educated speakers of English would go the 'but' route.

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Having said that, Yeats rather enjoyed Irish pronunciations, so quite possibly he might have preferred something more bullish. But that needn't bother us ... after all, we wouldn't say 'Oonder Ben Boolben'.