Well yes it's always easier to have the English as the enemies because:
1) It's very fashionable in revisionist circles to see England as some sort of anachronistic imperialist blob which needs no motivation at all to invade somewhere and be evil. Very convenient for a lazy plotline and to ensure you don't offend anyone.
2) American audiences have learnt to associate the RP English accent as belonging to the most convincing villain.
So in the base game we got the Saxons crudely mushed into "England", which somehow manage to be the villains despite the fact that the main character's faction is ra***g and pillaging all their towns and cities. et sic vadit.
Anyway this DLC has a nifty petty King of Ireland based on a real Ua Neill King which is pretty nice. Dublin of course was settled by the Vikings, who to varying degrees assimilated, and at this point in time still control it. To what extent the story will accurately portray Ireland in this period as a fractured assortment of tribes, with a titular "king" which alternated between the two Ua Neill tribes, remains to be seen. That Eivor will help him is not entirely implausible. Various Norse warlords allied themselves (at a steep price) with various Irish tribal lords to exploit internal divisions within Ireland to help them gain advantage over their own Norse rivals. But yeah, he/she will obviously be doing it for some noble reason, which will inevitably be quite silly.