The Gaelic social order rested on two interlocking units. The fine was an extended kin-group descended through male lines from a common ancestor, with the close family -- the derbfine, those descended from a common great-grandfather -- forming the core. The tuath was the territorial unit, perhaps 700 to 3,000 people, ruled by a king (ri tuaithe) who was elected from within the ruling fine according to a system called tanistry. When a king took office, his successor (tanaiste, anglicised to 'tanist') was chosen in his lifetime from among eligible relatives. The system was extraordinarily fluid: ambitious cousins competed in war and law for the right to inherit; dynasties rose and fell. Brehon Law, the elaborate legal code maintained by a hereditary class of jurists called brithem, governed it all. Brehon Law allowed women significant rights of inheritance and divorce that would not return to Ireland until the late 20th century. It survived in fragments under English rule for centuries; its full corpus, the Senchas Mar, was set down by monks in the 7th and 8th centuries and runs to thousands of pages.

Pre-Christian Gaelic Ireland had a literate caste called the filid -- poet-historians who learned by oral repetition, took up to twelve years to qualify, and could destroy a king's reputation with a satire so devastating it was believed to raise physical blisters. Below them were the bards (lower poets), and below them the storytellers, the seanchaidhthe. When Christianity arrived in the 5th century -- traditionally with Palladius in 431 and then Patrick, though both figures are partly legendary -- the filid kept their privileges. The monks who arrived in their wake established a literary tradition that would astonish Europe. The Book of Kells, made around AD 800 in either Iona or Kells, is the most spectacular illuminated manuscript surviving from the early medieval West. Saint Columba carried Gaelic Christianity to Iona in 563; Columbanus took it to the Frankish kingdoms in 590; in the 7th and 8th centuries, when most of Europe had forgotten how to read Greek, Irish monks were teaching it. The historian Thomas Cahill's bestseller How the Irish Saved Civilisation overstates the case, but only slightly.

By the 9th century, Viking longships were burning Irish monasteries up and down the coast. Within a generation, the Vikings had become permanent: Dublin was founded by them in 841, with Wexford, Waterford, Cork and Limerick following. These were Ireland's first towns. The Irish absorbed them as the Norse-Gaels (Gall-Goidil). The most famous figure of the period was Brian Boru, born around 941 in Munster, who fought his way upward through Viking and Irish enemies alike, was acknowledged as High King by Mael Sechnaill in 1002, and styled himself Imperator Scottorum -- Emperor of the Gaels. On Good Friday 1014, at the Battle of Clontarf just north of Dublin, Brian's forces defeated a coalition of Vikings and rebellious Leinster Irish. Brian himself was killed in his tent at age 73. The battle is often portrayed as the day the Irish drove the Vikings out, but the truth is messier: the Vikings stayed, integrated, and continued to dominate Dublin's trade. What Brian's death really ended was the chance of a unified Gaelic monarchy. The High Kingship fragmented again. A century later, the Normans were able to walk in.

In 1166, the deposed King of Leinster, Diarmait Mac Murchada, fled to England and persuaded Henry II to send troops to help him reclaim his throne. The Norman expeditionary force that landed in 1169 -- led by Richard de Clare, 2nd Earl of Pembroke, known as Strongbow -- changed Ireland forever. Strongbow married Diarmait's daughter Aoife, took the kingship of Leinster, and Henry II had to come over personally in 1171 to make sure his vassal did not establish an independent Norman-Irish state. The Lordship of Ireland was created. Yet the Norman conquest stalled. Outside a fortified zone around Dublin and the south-east -- the area that became known as 'the Pale' -- Gaelic chieftains continued to rule by Brehon law, speak Irish, and ignore the English crown. Many of the Norman lords themselves, marrying Irish women and learning the language, became (in the famous phrase of an English official) 'more Irish than the Irish themselves'. The Statutes of Kilkenny in 1366 tried desperately to legislate against this Gaelicisation, banning intermarriage and forbidding the English colonists to speak Irish. They failed. For three more centuries, the Gaelic order endured.

The end came in stages. Henry VIII declared himself King of Ireland (not just Lord) in 1542 and began enforcing English law and Protestant religion on a Catholic Gaelic-speaking population. The Elizabethan conquest of Munster in the 1580s broke the old Desmond Geraldines; the Nine Years' War in Ulster ended at the Battle of Kinsale on Christmas Eve 1601, where Hugh O'Neill and Hugh O'Donnell's combined Gaelic forces, joined by a Spanish expedition, were crushed by an English army under Lord Mountjoy. On 14 September 1607, O'Neill, O'Donnell's brother Rory, and ninety leading members of the Gaelic Ulster nobility boarded a French ship at Rathmullan on Lough Swilly and sailed away. They went to Rome and never returned. With their departure, the Gaelic political order ended. The land was confiscated and given to Protestant settlers from Scotland and England -- the Plantation of Ulster, whose consequences are still being negotiated four centuries later. The Irish language, Brehon law, the bardic schools, the tuatha -- all began their long retreat. They did not disappear. The Irish language still survives in the Gaeltacht regions of the west. Gaelic football and hurling are still played on Sunday afternoons. The names on the map -- Loch Garman, Corcaigh, Doire, Dun Laoghaire -- are still Gaelic words. But the order that produced them ended one autumn afternoon on a small Donegal pier.