Not even St. Patrick himself could protect Ireland from the Vikings. When the Nordic raiders launched their first attack on Ireland in 795 A.D. by raiding an island monastery, Irish monks wielded prayers in self-defense. No heavenly intercession arrived, however, to save their cloister from being sacked.

The Vikings continued to stage small hit-and-run raids on unprotected coastal monasteries before sailing up the River Shannon in the 830s to plunder inland religious settlements. The foreign invaders even defiled Ireland’s holiest turf by plundering the monastery of St. Patrick at Armagh, slaughtering its monks and desecrating the buildings erected in honor of Ireland’s patron saint.

While it was the Danes who attacked England and Francia, it was mostly the Norwegians who raided Ireland. By the 840s, those Vikings began to establish permanent ship bases along the coastline from which they could plunder year-round. Coastal enclaves at Dublin, Wexford, Waterford, Cork and Limerick became trading centers for Ireland’s treasures—and its people.

According to John Haywood, who chronicles the exploits of the Scandinavian raiders on four continents in his new book, “Northmen: The Viking Saga AD 793-1241,” slavery had been rare in pre-Viking Ireland, but that all changed with the arrival of the Norwegians. “The Vikings milked Ireland fairly systematically for a couple hundred years for slaves. Dublin originated really as a slave market,” he says.

At first glance, Ireland appeared to be ripe for a complete Viking conquest. After all, the Vikings took advantage of internal divisions to seize England and Francia, and Ireland was the most politically fractured country in Western Europe, according to Haywood. A complex hierarchy of 150 local kingdoms and a dozen over-kingdoms ruled the island, and even high kings only directly ruled over small territories. Furthermore, Irish kings often welcomed the incursions of the foreign invaders as a means to weaken their domestic rivals.

Yet, while the Vikings had success raiding Ireland, they failed to conquer the island as they did other lands in Europe. “It looks like there could be a Viking takeover, but it’s pretty clear by the middle of the 10th century that apart from fortified enclaves along the coast, they have failed absolutely to control territory in Ireland in the same way as they did in England, Scotland, France or Russia. If they don’t have forts around them, the Vikings can’t really survive in Ireland.”

Haywood says Ireland’s decentralized system of governance, which made the island appear vulnerable to a larger force, actually had the opposite effect in protecting it from a Viking takeover. He points out the the centralized kingdoms of early medieval Europe were the most easily conquered since there was a much smaller ruling class to either eliminate or negotiate with to forge a lasting peace agreement. With so many kings to subdue and bargain with in Ireland, it proved much more difficult to eliminate or co-opt the existing power brokers.

“In many ways one of the most fascinating aspects of the Irish is how this apparently highly divided island proved much more effective at combating the Vikings than England did,” Haywood tells HISTORY. “In Ireland you could kill a king, but it doesn’t really disable the kingdom because there are an infinite number of successors. Also, with all these Irish kingdoms, you can’t negotiate a permanent peace with anybody. In England in the 9th century, by contrast, most of the kingdoms are quite centralized and succession to the throne is confined to a small group of people. So if you can knock out a royal family, it’s leaderless and you can take over.”

“On the face of it, Ireland looks like a place that the Vikings should be able to rampage over, but it’s deceptively strong,” Haywood says. “So the Vikings can raid Ireland, but they can’t hold any territory. They can only take what they can carry back to their fortress cities. The Irish position is quite remarkable in that their political structure makes them look weak, but in effect it makes them unconquerable.”

Norse influence in Ireland began to wane by the time of the rise of the legendary king Brian Boru who sacked the Viking town of Limerick in 968 and eventually became the overlord of Cork, Wexford and Waterford. On Good Friday in 1014, the high king’s army routed the Vikings and their allies at the Battle of Clontarf outside of Dublin, although a small group of Norseman managed after the battle to kill the elderly king as he prayed in his tent.

The Vikings remained in Ireland after agreeing to pay a tribute. By that point, however, they had been fully integrated into Irish life, and most of them had converted to Christianity. The Viking Age in Ireland didn’t come to a definitive close until the Norman invasion in the 1170s and the last Norse king of Dublin fled to the Orkney Islands.

Haywood says that in spite of the centuries-long presence of the Vikings, their impact on Ireland was surprisingly slight outside of their influence on Irish art styles and weaponry and their opening up of the island to European trade as a byproduct of the slave trafficking. Among the few remnants of the Viking Age that still loom over the Irish landscape today are the tall, slender round towers that were built by monks at monasteries to provide refuge from the Scandinavian raiders. Over 80 of the stone towers were known to have been built at places such as Cashel and Glendalough, and the tallest surviving round tower can still be visited at Kilmacduagh in County Galway.