I don't have any book sources for this that I know of, but I took a class on the history of Ireland and the professor actually brought this derogatory term up in one of the lectures. 'Hun' was a common nickname/insult used when discussing Germans during WWI. The name actually doesn't have anything to do with ethnic connections, but refers more to a perception of the German ideology. The insult thus compares the German Empire, and through that refers to all Germans, to the Huns of ancient times, who ravaged many regions under Attila. Basically, it insinuated that Germans were violent barbarians. The population of England, which self-identifies as Germanic through the Anglo-Saxons, were (and still are a bit) at odds with many inhabitants of Ireland who identify as ethnically Gaelic. So to answer your question explicitly the Irish are saying that the English, like the Germans that they are related to, are violent barbarians. It's just a racial slur, essentially.

http://en.wikipedia.org/wiki/List_of_terms_used_for_Germans#Hun (This goes into finer detail about the term as it was used to refer to Germans, including the exact origin of the term which includes a speech by Emperor Wilhelm II in 1900)

Didn't the British also refer to the Germans as Huns? It may be a catch-all term for "the enemy".

I've never seen a scholarly discussion of it. I've also never seen it in 1920s sources. It is a common term of abuse by Celtic supporters towards Rangers and their fans but I don't know the origins of it in that.