The Neolithic (4500 BC - 2500 BC)

The Neolithic saw the introduction of farming and pottery, and the use of more advanced stone implements. It was once thought that these innovations were introduced by a new wave of settlers, but there is no compelling evidence for a large-scale invasion at this point in Irish history. It is much more likely that the Neolithic revolution was a long and slow process resulting from trade and overseas contacts with agricultural communities in Britain and on the continent.
Agriculture began around 4500 BC. Sheep, goats, cattle and cereals were imported from Britain and the continent, and the population rose significantly. At the Céide Fields in County Mayo, an extensive Neolithic field system - arguably the oldest in the world - has been preserved beneath a blanket of peat. Consisting of small fields separated from one another by dry-stone walls, the Céide Fields were farmed for several centuries between 3500 and 3000 BC. Wheat and barley were the principal crops cultivated.
Pottery made its appearance around the same time as agriculture. Ware similar to that found in northern Britain has been excavated in Ulster (Lyle's Hill pottery) and in Limerick. Typical of this ware are wide-mouthed, round-bottomed bowls.
But the most striking characteristic of the Neolithic in Ireland was the sudden appearance and dramatic proliferation of megalithic monuments. The largest of these tombs were clearly places of religious and ceremonial importance to the Neolithic population. In most of the tombs that have been excavated human remains - usually, but not always, cremated - have been found. Grave goods - pottery, arrowheads, beads, pendants, axes, etc - have also been uncovered.

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.