Not to be left out, small groups of Vikings started showing up in Ireland around 800 AD. At first, they were more interested in raiding the local monastery than making the island their permanent home.
In response to the Viking raids, the Monasteries build tall round towers, with entrance doors high up above the ground. The Monks would use the towers to watch for the Viking raiders and should the Vikings arrive, the tower became the Monks safe refuge.
In 914 AD the Vikings decided it was time to take the island in earnest, and a true invasion conmence.