The Mesolithic

What little is known of pre-Christian Ireland comes from a few references in Roman writings, Irish poetry and myth, and archaeology. During the Pleistocene ice age, Ireland was extensively glaciated. Ice sheets more than 300 metres thick scoured the landscape, pulverizing rock and bone, and eradicating all evidence of early human settlements. Something similar happened in Britain, where human remains predating the last glaciation have been uncovered only in the extreme south of the country, which largely escaped the advancing ice sheets. During the last glacial maximum (circa 16,000 BC), Ireland was an arctic wasteland, or tundra. The Midland General Glaciation covered about two thirds of the country with a drifting sheet of ice. It is highly unlikely that there were any humans in the country at this time, though the possibility cannot be discounted entirely.
The earliest evidence of human occupation after the retreat of the ice has been dated to between 8000 and 7000 BC. Settlements of Mesolithic hunter-gatherers have been found at about half a dozen sites scattered throughout the country: Mount Sandel in County Derry; Woodpark in County Sligo; the Shannon estuary; Lough Boora in County Offaly; the Curran in County Antrim; and a number of locations in Munster. It is thought that these settlers first colonised the northeast of the country from Scotland. Although sea levels were still lower than they are today, Ireland was probably already an island by the time the first settlers arrived by boat. There is nothing surprising in this, though, for most of the Mesolithic sites in Ireland are coastal settlements. Clearly, the earliest inhabitants of this country were seafarers who depended for much of their livelihood upon the sea. In some ways this economy was forced upon them, for many centuries were to pass before the treeless permafrost was transformed into a densely forested fertile land.
The hunter-gatherers of the Mesolithic era lived on a diet of seafood, birds, wild boar, deer and hazelnuts. They hunted with spears, arrows and harpoons tipped with small flint blades called microliths, while supplementing their diet with gathered nuts, fruit and berries. They lived in seasonal shelters, which they constructed by stretching animal skins over simple wooden frames. They had outdoor hearths for cooking their food, and they are known to have built canoes from dug-out tree trunks.
During the Mesolithic the population of Ireland was probably never more than a few thousand.