The Viking threat, the adoption of new weapons technology, the establishment of cities, and the development of seafaring trade brought great changes to Ireland, and the Vikings were not able to establish themselves in Ireland for long, but their culture and language left a mark on Ireland.

Many of the settlements of the Kingdom of Ireland at that time were a mixture of the original Irish and Vikings, and many of the kings of Ireland at that time were named after Vikings, and some place names also contained elements of Viking names, and many cities on the Irish coast today also reflect the structure of Viking cities.

For more than a hundred years, the Vikings did not coordinate their attacks, and the Irish defenses were inconsistent, and there were constant power struggles within Ireland, with these internal factions sometimes allied with the Vikings, and finally the Tara family in the northeast prevailed, and they also succumbed to the Vikings who settled in Ireland.

At the end of the 10th century, Brian Beaujou largely unified Ireland, and in 1014 he also defeated the Vikings, who became known as Brian Beaujou, King of Ireland, a wise monarch most likely to unify Ireland and establish a centralized state.

Brian Bojou (. 941–23 April 1014) was King of Ireland, ending the high degree of fragmentation of Irish royal rule and establishing centralized rule in Ireland.

Building on his father's achievements, especially those of his older brother, Brian. Beaujou himself became King of Munster, then conquered Leinster and eventually became King of Ireland, the founder of the Cauchy dynasty.

With a population of 500,000 people, Ireland has more than 150 kings, larger or smaller domains.

At that time, Brian Bojou's fortune was only a sub-kingdom of the Munster Kingdom, equivalent to a land the size of an earldom, and he fought in the north and south, and captured the Kingdom of Desmond in ten years, unifying the entire territory of the Munster Kingdom.

Bryan Beaujou then led his army to annex Dublin and the surrounding small kingdoms, so that he controlled most of central and southern Ireland for a hundred years, and the entire land on the south bank of the Shanon River was in Brian Beauru's hands.

However, Brian made it clear that his ambitions were not satisfied by the compromise of 997, and in 1000 AD he led the Munster-Dublin coalition against the High King of the North, King of Connaught, in a struggle over who would control all of Ireland restarted.

Since the territory of the Northern Coalition was separated by the Shannon River, Brian's naval forces were free to attack the coasts on either side of Connaught and his allies.

The outcome of this war is unknown, but according to the chronicles, in 1002 A.D., King Connaught gave his title to Brian, and although they do not say how it came about or why it happened, it is enough to show that Brian Bojou succeeded in defeating many powerful enemies in the north.

Later historians provide a story in which Brian challenged the High King, King Connaught, in a battle on Mount Tara in the province of Meath, where the High King King Connaught asked for a one-month truce so that he could mobilize his forces, and Brian agreed to his request.

When the deadline came, he did not gather the regional rulers who were nominally subordinate to him, and he was forced to cede his title to Brian.

Given Brian's style of entering the war, some have expressed skepticism about this interpretation. If he finds his opponent at a disadvantage, there is a good chance that he will make the most of it rather than give his enemies time to challenge, and besides, given the length and intensity of the struggle between Mahler Széchener and Brian, it seems unlikely that the king would give up his title without a fight.

It is widely believed that in 1002, Brian became the new king of Ireland as a veritable supreme king. Unlike people who have had this title before, Brian wants to be the supreme king.

In order to achieve this, he needed to impose his will on the local ruler of the only province that had not yet recognized his authority, the Ulster.

The geographical location of the Alster presents a huge challenge. There are three main routes through which invading armies can enter the province, all three of which are beneficial to the defenders. Brian first had to find a way to bypass these defensive "bottlenecks", and then he had to subdue the king of those extremely independent regions, Ulster.

Brian spent 10 years campaigning to achieve his goal, and considering that he could and did call on the armies of the rest of Ireland to join the war, it showed how strong King Ulster was. Once again, Brian's coordinated use of land and sea allowed him to win.

While the rulers of Ulster could bring the advance of Brian's army to a halt, they could not stop his fleet from attacking the shores of their kingdom. When Brian entered the province of Ulster, he systematically defeated all the regional rulers who rebelled against him, forcing them to recognize him as their overlord.

It was in the process that Brian resorted to another means of consolidating his control, not just in the province of Ulster, but throughout Ireland.

Unlike elsewhere, the center of the Church of Ireland is not the bishop of the episcopal diocese and the bishop of the archbishopric, but the monastery led by the powerful Abu people, who were members of the royal dynasty and whose monasteries resided there. One of the most important monasteries is Alma, located in the province of Ulster.

According to historical records, in 1005 AD, Brian donated 22 ounces of gold to the monastery and declared that Alma was the religious capital of Ireland and that all the other monasteries should send the money they had collected there. This was a smart move, as the supremacy of the Monastery of Alma would only last so long, as Brian remained the supreme king.

Therefore, in order to support all of Brian's wealth and power, it is in Alma's interest.

Interestingly, Brian does not mention "Adere", i.e., "noble king", in the Book of Alma, but rather says that he is "the emperor of Scotland".

Although only speculation has been suggested, it has been suggested that Brian and the Church of Ireland together sought to establish a new form of royal power in Ireland, one that was modeled after the struggle for dominance brought about by England and France, where there were no smaller regional kings, but only one king (or emperor) over all the princes and nobles in a centralized state.

Unfortunately, on Easter Friday, 1014 AD, during the Battle of Clontarf, Brian Bojou led his army to heroically repel the Viking invasion. However, due to the death of Bojou himself and his lineage, the royal family had no successor, and the tribal leaders of various regions rose up to seize power for themselves, so that the chaos in Ireland has continued to this day.

Brian Beaujou's descendants still rule the Duchy of Munster and still have this enormous influence, and the Irish will still unite against the Norman invasion under the threat of the Normans, just as they did against the Vikings a hundred years ago.

This really made everyone present a little worried, not only them, but even William frowned when he heard it.