Potentially United Ireland

In the year 1010 historically, an Irish ruler named Brian Boru united almost the entire island. He subjugated the major Irish fiefdoms. However, in 1014, he was defeated by Vikings outside of Dublin in the Battle of Clontarf. It was a close battle and a vassal of Boru's left before the battle. Once he died, the kingdom collapsed and Ireland wasn't united until the British invaded later on.

What if he won the battle? I feel like this Alternate History would be interesting; here is one theory:

After the slaughter at Clontarf, Boru marched victoriously into Dublin, annexing the region. Being 71 at this time, he died soon after and pretenders would most likely rise due to the fact that it was a medieval kingdom. The only way that the kingdom would survive would be if his son, Dungal, succeeded.

Throughout Dungal's reign, the remaining Viking settlements, including Cork, would be annexed and Irish power would grow over time under a centralized authority. Similar to Scotland at the time, Ireland resisted England from complete annexation.

At this point, it's difficult to predict what could've happened. Scotland was eventually annexed by Great Britain and lost many wars against England but that was because of inheritance, not a direct invasion. How would Ireland be treated under a growing English empire from the fourteenth century onwards, especially during the Hundred Years War? As said before, it's difficult to predict but what does everyone think?

A United Ireland would probably lead to greater defense against English invasion but i do think at some point probably much later on that Ireland would either be absorbed or Vassalized into the UK. I cant see such a pair of nations to exist at the same time and place in the Medieval British Isles. Though i do think do to Ireland being United it wouldn't be so integrated into the UK in fear of Irish Rebellion and with that the English probably would either leave earlier then in Our Timeline or face some sort of large Rebellion by the Irish. Ireland in that timeline would probably also hold Northern Ireland due to less Protestation of the Ulster Region but that could depend on how fast that would or not occur in this Alternate Timeline.

Well, perhaps a united Ireland would team up with Scotland and France against the English. Then the English might actually have a run for their money.

Expect a lot of dynastic mingling with Scottish, and perhaps French dynasties in a bid to protect themselves from English invasion. I can also see a potential "Greater Auld Alliance" including Ireland to support the Scots militarily, while the French can focus on English holdings in France, maybe concluding the Hundred Years War early. I wouldn't expect the Scots and Irish to continue to hold out forever though, they would be grouped under the English Banner eventually.

Welp it would definitely change the irish culture and language forever the irish language and culture would have developed differently from our timeline with the irish population preserving their language if they formed an alliance with their celtic brothers like scotland against the english england would have never became a major power in europe without securing the british islands. ireland could potentially become a colonial power too without the english the irish people would have colonized north america too at the end ireland would have became a great power in the world but i think that our world would have been completely different without england becoming a great power and celts invading their homeland all the time the protestant reforms in england would have never happened which means that the industrial revolution wouldn't happen.

I wonder what effects this would have on the reformation in Ireland. Ireland stayed loyal to the church mainly because the reformation was meant to be an attack on the ruling hierarchy, but in Ireland it failed because the Irish used Catholicism as a means of identification over the ruling Protestants. If their identity at the time of the reformation did not so heavily rely on the Catholic Church, but more on their own Monarch due to being unified so early on, how would the reformation then effect Ireland?