In the 10th century A.D. the Vikings began to raid Ireland. They conquered Irish settlements, plundered and burned monasteries, and imposed their culture and language. Ireland’s educational attainment began to decline and the clergy lost their former importance.
The influence of the Vikings was not only negative. The conquerors built many new towns in rural Ireland, including the first ports. For example, it was the Vikings who founded Dublin and Limerick. The appearance of the ports stimulated maritime trade. The Vikings taught the locals how to sail and enriched the Irish language with nautical terms.
In the early 11th century, the Irish kingdoms united under the leadership of King Brien Boru. He defeated the Vikings at the Battle of Clontarf in 1014, but was himself killed. The conquerors were driven from the island, but the king’s death caused Ireland to break up again into smaller kingdoms: the Midlands, Munster, Ulster, Connacht and Leinster.
Ireland retained a supreme king, but his power was limited. The kingdoms fought constant internecine wars among themselves. This was bad for the development of the country. The Irish lagged behind their neighbors, the British, so they could not withstand their later onslaughts.