Once upon a time, Ireland was besieged by hordes of conquering Vikings. The Viking campaign to claim the island were not successful, but the story is an epic one.
The Vikings launched their first offensive against the Irish in 795 A.D. A raiding party attacked a monastery that was completely undefended. The monks that lived there were, obviously, ill-prepared to handle a Viking raiding party. The Vikings continued their coastal campaign, terrorizing monasteries and eventually traveling up the River Shannon to attack inland strongholds around 830 A.D. They mostly targeted religious sites. Most notably, they destroyed the monastery of St. Patrick at Armagh.
About a decade later, the Vikings had entrenched themselves along the Irish coast, building ship bases that served as launching points for raids.
The arrival of Norwegians to the island brought more than just plunder and pillage. It also introduced slavery, which had been very uncommon before. Ireland served as a feeder population for the Viking slave trade for about two hundred years.
Ireland was made extremely vulnerable to outside invasion by its balkanized internal politics. It was really a collection of fiefdoms, governed by about 150 local kingdoms that were subservient to twelve larger kingdoms. And many Irish kings were actually amenable to invasion by foreigners, in the hopes that it would weaken their Irish adversaries.
The Vikings were never, however, fully successful in their attempt to claim Ireland completely. The failure was an exception to the rule - the Vikings were able to conquer England, France, Scotland and even Russia. But the small island state got the better of them.
It may have been largely thanks to Ireland's fractured governance that the island was able to weather the storm long-term. Conquered states on the mainland had more centralized systems of rule, making it much easier to unseat existing power. But the profligate kings in Ireland, all of whom would need to be subdued by treaty or by force, may have proved beyond the Vikings' means to handle.
Viking power in Ireland stuttered and was then snuffed out by the ascendancy of king Brian Boru. Boru routed the Vikings from Limerick in 968 and then defeated the Viking army at the Battle of Clontarf. The confrontation would prove fatal both to the Irish Viking kingdom and to Boru himself, who was assassinated in his tent by a contingent of surviving Vikings after the battle.
The Norse remained in Ireland, but with power sublimated to Irish rule. Most remaining Vikings converted to Christianity and paid tribute to the Irish crown. 1170 saw one last-gasp attempt to exert Viking control over the island, but the Norse king of Dublin was defeated and retreated to the Orkney Islands.