Ireland was one of the last places in Europe to be settled by humans. The persistence of the last Ice Age meant that the first Mesolithic [Middle stone Age] hunter-gatherers could not have arrived in Ireland until, at the earliest, 7500 BC. Rising seas had almost certainly swept away the last land connection(s) with the Continental peninsula later to become Britain. Tundra conditions probably prevailed, but pinewoods and great forests of oak, elm and ash were already beginning to grow. The most ancient habitation site in Ireland discovered so far is about 7,000 years old.
Neolithic [Late Stone Age] agriculturists began arriving in large numbers after 4000 BC, importing domesticated cattle, sheep and goats, and started clearing highland forest and cultivating cereals. Some lived in large communities, as can be seen at the Céide Fields site in Co. Mayo.
Megaliths (great stone monuments) from the Neolithic period (c.4000 – c.2000 BC) vary considerably in size, shape and apparent or possible functions.
Cromlechs or dolmens are usually considered to be portal tombs or aboveground burial chambers, although their east-sloping slabs suggest some kind of sun-worship, perhaps involving sacrifices.
Court-cairns may have served as cult centres for a scattered population.
The best-known tumuli or passage-cairns are located in the of the Boyne River Valley . They are older than Stonehenge in England or the Giza pyramids in Egypt, and clearly the work of a well-organised and advanced civilisation.
At Newgrange, a massive stone mound dated to 3200 BC, a passage leads to a central chamber, which is only illuminated by sunlight at dawn on 21st December, the winter solstice and shortest day in the year.
Throughout the Early Bronze Age (c.2200 BC – c.1200 BC) Ireland had a flourishing metal industry, and ornate copper, bronze and gold objects were exported widely to Britain, the Continent and perhaps farther afield. The people built wedge tombs, similar to those in Brittany, and stone circles, thought to have been used for religious rituals. They lived in groups of wood-framed huts that were enclosed within a stone wall or earthen bank, commonly known today as ring-forts, or on artificial palisaded lake islands called crannogs. Their main means of living was through farming. They grew crops of wheat and barley and herded their own cattle, sheep and goats. Meat was cooked for feasts in huge bronze cauldrons over fires.
The Late Bronze Age (c.1200 BC – c.500 BC) is strongly identified with the appearance of the first “hill forts” or Dúns, usually enclosing beehive huts, essentially stone igloos, which provided shelter for small families, and larger dwellings built like upturned boats. Among the most impressive constructions of this period are Dún Beg on the Dingle Peninsula and Dún Aengus on the Aran Islands.
The Iron Age in Europe is associated with the arrival of Celtic culture(s) in Ireland, due to a steady infiltration of people from Britain and the European mainland over several centuries.
The first Celtic influences appear to have reached Ireland as early as 700 BC. Tribes wielding iron weapons must have had a great advantage over those only possessing bronze. Horses also arrived in Ireland at around this time, providing the Celtic cultured people with increased mobility and an element of terror. Long earthworks such as the Dorsey in Co. Armagh and the Black Pig’s Dyke, which runs across much of lower Ulster, date from this period, and were probably intended to prevent cattle raids.
La Tène culture (named after a Celtic site in modern Switzerland), distinguished by an art style of abstract geometric designs and stylised bird and animal forms, found on distinctive metalwork and stone sculpture in many parts of Europe, may date in Ireland from 300 BC or earlier. La Tène artefacts, such as gold torques, collars, bracelets and other personal adornments as well as swords, scabbards, shields and trumpets for use in battle, have been found in Ulster and Connacht. These are areas associated with many ancient legends such as those of Cú Chullainn and Conchobar mac Nessa. It appears that these stories may well reflect some kind of historical reality, especially in relation to tales of the Ulaid warriors, whose capital was Emain Macha (Navan fort, near Armagh).