Archaeologists believe that the first human settlements in Ireland were made relatively late in European prehistory, about 6000 B.C.

Ireland's original inhabitants were classic Middle Stone Age (Mesolithic) hunter-gatherers who used stone implements. Later, about 3000 B.C., they evolved into classic Bronze Age (Neolithic) people who cultivated crops, raised domestic animals and (even though metals were extremely rare in Ireland) made weapons, tools and jewelry out of bronze. At the same time they fine tuned their stone implements (such as stone axes that were mass produced and traded abroad). Starting about 2000 B.C., they built the massive stone sanctuaries and tombs ("megaliths") that still dot the countryside. By the 1st Century B.C., Ireland (as well as Scotland) was under the control of a race called the Picts, a Neolithic people described in Irish folklore as the "Fir Bolg".

Then came the Celts, and nothing has had a greater impact on Ireland. They dominated the entire Island until 1170 A.D., and further dominated major parts of it until 1600 A.D. The first Celts -- probably a small migration -- arrived in Ireland about 600 B.C., bringing Iron Age skills with them. A major migration arrived about 350 B.C., and it is certain that the Celts were well established throughout the Island by 150 B.C.

The Celts originated in central Europe, but at an early date expanded into southern France and northern Spain. They were fair of skin, red-blond of hair, taller and larger than their contemporaries. Their language was a branch of the Indo-Germanic languages that also includes German, Latin, Slavonic and Persian. Eventually, there evolved several dialects of Celtic, including the dialect of the so-called Q-Celts [a.k.a. C-Celts], which eventually prevailed, and that of the P-Celts. The Celts were bound together by a common culture and a common language, but otherwise probably were nothing more than a loose confederation of largely autonomous tribal units, prone to tribal warfare with one another.

The central European Celts were a formidable military force. They used iron weapons, and were fierce in battle. They dominated central and Western Europe early in the 1st Century B.C., sacked Rome in 390 B.C., raided Delphi a century later, and founded the kingdom of Galatia in Asia Minor.

Archaeological evidence indicates the Celts arrived in Ireland in two major waves: (1) One wave, probably Q-Celts [a.k.a. C-Celts], came directly (by sea) from the Continent (southern France, and/or northern Spain) to southwestern Ireland, and (2) The second wave, probably Q-Celts but perhaps P-Celts, traveled from France and the mouth of the Rhine first to northern Britain, and thereafter to northeast Ireland. In addition to the major migrations, there would appear to have been virtually continuous smaller migrations of Celts from the Continent to Ireland; this would explain how artifacts reflecting the La Tien culture of 5th Century B.C. Switzerland were present in 2nd Century B.C. Ireland.

The scholarly evidence is reasonably consistent with Irish folklore, which holds that the three sons of King Mileadh of northern Spain -- namely Heremon, Heber and Ir, together "the Milesians" after King Mileadh -- invaded Ireland about the time of Alexander the Great (356-23 B.C.) and vanquished the Tuatha De Danan, who previously had gained superiority over the Fir Bolg (Picts) and the Fomorians. Virtually all of the later Gaelic rulers claimed to be descendants of Heremon or Heber.

These new arrivals in Ireland at some point began to call themselves "Gaels", a term they limited to inhabitants of Ireland who shared their ethnicity. They probably never called themselves "Celts", which was the name given to them and to their language by later scholars, who used the terms to encompass all people (including Scots and other non-Irish) who shared that culture and language.

Politically, the Celts divided Ireland into four provinces: Leinster, Munster, Ulster and Connacht. Even before the Celts, the basic units of Irish society were the tuatha, or petty kingdoms, each of which was quite small, perhaps 150 tuatha for a population of less than 500,000. This societal structure perfectly suited the newly arrived Celts, who were predisposed -- by their ancient culture, presumably, or was it in the genes -- towards relatively small and autonomous tribal units. Indeed, a recurring theme throughout Irish history is popular resistance to strong centralized government under a strong monarch, and insistence instead upon a loose confederation of small and autonomous units of government.

Although the tuatha were autonomous, the people shared a common language (Q-Celtic) and religion (Druidism), plus a remarkably uniform rural society -- there were no towns or villages -- which included a variety of social classes, including the Brehons (judges and lawmakers), Fili (professional poets or scholars, custodians of oral history) and Druids (priests). The two principal classes were free and landed nobles, and unfreeze peasants (slaves, laborers, and workingmen). Nobles and peasants alike passionately loved their land, and this evolved into the ancient Brehon law of gravelkind, under which land was the common property of society, subject to the preferential (but not permanent) rights of families who worked or lived on it; and although the petit king nominally "owned" it, he did so only as trustee, i.e., he could not transfer it. Under another Brehon law, tanistry, during the tenure of any particular king, his successor was determined, and the selection was made not through inheritance, but rather by consensus or election from within a group of extended royal families that included all relations in the male line of descent for four or five generations.

In about 200 AD, Conn Ced-cathach formed a monarchy in the central parts of the island (of which the eastern part was called the "middle kingdom"), while his southern rival, Eoghan Mor, established the kingdom of Munster. Eventually, the two came to an agreement dividing the island along a line from Dublin to Galway into a northern part ("Conn's Half") and a southern part ("Mogh's Half"), a division that lasted until Brian Boru in 1002.

Conn's descendant, Niall of the Nine Hostages (r. 380-405 AD), is generally regarded as the first Ard Ri (high king) of Ireland, but it is doubtful that his commands were followed everywhere on the island. Although no one was more powerful, Niall's position should be compared to that of presiding officer of a voluntary confederation of petit kings. He ruled from Tara, and claimed to be descended from Heremon. Late in Niall's lifetime, two of his sons -- Eoghan (who was the older of the two) and Conall Gulban -- conquered northwest Ulster in about 400 AD, and founded a new kingdom. Another of Niall's sons, Laeghaire, succeeded him as Ard Ri, while still other sons established kingdoms elsewhere on the Island. Niall's descendants, who were called the Ui Neill, continued to dominate Ireland until the emergence of Brian Boru in 999.