[The Squatter of Amon Rûdh]

I agree with Gwaihir. Eriador is not the British isles, although the British countryside was undoubtedly a strong influence on many of Tolkien's locations, which isn't terribly surprising given that he lived there. There are fields of barrows all over Britain, any combination of which could have been the inspiration for the Barrow Downs, which location stands for nothing more than an eerie collection of ancient graves, some of them inhabited by the shades of the dead. I wouldn't be surprised if Tolkien had wandered around a few barrows in his time: they can inspire that sort of thinking.

As for the idea that Arnor is England and Angmar some bizarre amalgam of Scotland and Ireland, or vice-versa, I think that highly unlikely. Setting aside the fact that Tolkien disagreed with allegory on principle, the English regard those old medieval wars as ancient history. Tolkien wouldn't have given them a second thought, let alone tried to demonise one side or the other. I mean to say, if he made no references to either of the massive global conflicts that took place in his lifetime, how likely is it that he'd refer to a couple of dynastic quarrels that took place long before he was born?

No, I'm afraid that Arnor versus Angmar is a simple matter of good versus evil. Despite whatever impression Hollywood might have created, there has never been such a war in Britain.