Sharp and his contemporaries found an English folk song culture which was very much alive in melodic invention - the tunes they wrote down were very different from anything you could have found in print in Chappell's books. And that process continued much longer in North America.

I get the impression that the English-language Irish song culture was pretty much dead as far as melodic invention went at the same time, and hasn't shown any signs of coming back to life since. Jim never mentions tunes at all - when he finds interesting current material in rural Ireland, it's all about verbal content. So I guess they just rehash a small fixed repertoire of commonplace tunes.

What does Roud say about the evolution of melodies?