Quaternary Glaciation
During the most recent Quaternary glaciation, ice sheets more than 3,000 m (9,800 ft) thick scoured the landscape of Ireland, pulverizing rock and bone, and eradicating any possible evidence of early human settlements during the Glenavian warm period (human remains pre-dating the last glaciation have been uncovered in the extreme south of Britain, which largely escaped the advancing ice sheets).
During the Late Glacial Maximum (ca. 13,000-10,000 years ago), Ireland was an Arctic wasteland, or tundra. The Midland General Glaciation ('Midlandian period') was originally thought to cover two thirds of the country with ice. Subsequent evidence from the past 50 years has shown this to be untrue and recent publications (Greenwood and Clark, 2009) suggest that ice went off the southern coast of Ireland. The early part of the Holocene had a climate that was inhospitable to most European animals and plants. Human occupation was unlikely, though fishing possible.
During the period between 17,500 and 12,000 years ago, a warming trend and a cool period allowed the rehabitation of northern areas of Europe by roaming hunter-gatherers. Genetic evidence suggests this reoccupation began in southwestern Europe and faunal remains suggest a refugium in Iberia that extended up into Southern France. The original attraction to the north during the pre-boreal period would be species like Reindeer and Aurochs. Some sites as far north as Sweden >10,000 years ago suggest that humans might have used glacial termini as places from which they hunted migratory game.
These factors and ecological changes brought humans to the edge of the Northernmost ice-free zones of Europe by the onset of the Holocene and this included regions close to Ireland.
There is no evidence that humans occupied Ireland at this time, but on the eastern side of the Irish Sea one site dated to 13,000 years ago was discovered that indicated people were in the area eating a marine diet including shellfish. It is possible that humans did occupy the region but found few resources outside of coastal shellfishing and acorns and did not continually occupy the region.
As the northern glaciers retreated, sea levels rose with water draining into an inland sea where the Irish Sea currently stands; the outflow of freshwater and eventual rise in sea level between the Irish and Celtic Seas inhibited the entry of flora and fauna from Europe via Britain.
Read more about this topic: Prehistoric Ireland