Or, perhaps, Americans act like entitled pricks when they enter his pub. Some Americans aren’t exactly good emissaries. I just got back from Europe and made an extra effort to tip well and be polite. I always got great service and everyone was very friendly and helpful.
In my experience at European eateries, museums, churches, etc. German tourists tend to be even louder than Americans. And Ireland is one place in Europe where Americans tend to be especially welcome. But as someone already pointed out, his business and his rules. Beyond that the owner can just go F himself.
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Of course you were treated well. You were spending money in their establishments. It doesn’t disprove my point however. Many Europeans do have an irrational hatred of America and Americans.