Level: EasyStart as High Chieftain Áed mac Gormlaith of Meath in 867, because he offers a good start and has casus belli to conquer all other Counties, but is at risk of being attacked by Vikings. You don't need to get all land in Ireland just enough to create the Kingdom. You have to be really quick and it may take some tries.
Alternatively, play the Rags to Riches scenario as Petty King Murchad of Munster. Fabricating claims is sufficient for this, as most of Ireland is divided among independent counts.
Credit to
Bayek von Kuwaq on Steam for his
guide. Information sourced from the
wiki.