I once managed to have quite a successful campaign as the Irish on Hard difficulty (mind you i was using the BKB Super Mod). Initially i took Iceland and Scotland, and then, after assuring naval superiority in the region, moved on to take over Scandinavia and then the Novgorodians, most of Russia soon falling into the hands of the Irish. Eventually, I managed to claim a significant chunk of the British Isles after the English were excommunicated. Mainly, i took my time and preferably tried to spring at rebel provinces that sprung up, especially early on. I think i still have the save file, i might play it later.