At some point after about 500 BC, there were certainly arrivals by Indo-European Celts (and perhaps even as early as 2000 BC), and they remained fully independent as Ireland was never conquered by the Romans.Instead the Celto-Irish helped to hasten the end of Roman control over Britain by constantly raiding the British coastline, capturing slaves and booty.
At various points in its early medieval history, from the eighth or ninth centuries onwards, Ireland was nominally united under the high kings (ard ri) and, but for many incursions by Danes, Normans and the Norman-dominated English, Ireland might have developed into a fully unified single kingdom in the same way as England had in the tenth century.
Trapped between this land bridge and the ice sheet to the north, the Irish Sea is filled by melt water that forms a vast lake.
At this time, the land bridge is finally submerged beneath the salt water of the Atlantic Sea.
Animals, including the Giant Deer, and the hunter-gatherers who have followed them are now cut off.
The land bridge makes a few more brief appearances as short-term fluctuations interfere with average sea levels before being swallowed up permanently.