The Laigin, or Dumnonii, were the third ethno-tribal group to come to Ireland, coming from Gaul shortly before the Gaels themselves, sometime during the first century B.C. Branches of the Dumnonii settled first in the Devon-Cornwall area before others moved on to Ireland. In southern Britain their kingdom gave its name to Devon (Dumnonia). There was a tribe called the Dumnonii, who inhabited most of south west Britain including Cornwall. Glastonbury, from ancient times had been an active seaport on the Severn estuary and situated to be the entrance point for a new religion into western Britain. An early Welsh story links Arthur to the Tor in an account of a face-off between Arthur and the Celtic king, Melwas, who had apparently kidnapped Arthur's wife Queen Guinevere. Avalon (probably from the Celtic word abal: apple) is a legendary island somewhere in the British Isles, famous for its beautiful apples and Lyonnesse is the sunken land believed in legend to lie off the Isles of Scilly, to the south-west of Cornwall.

In the time of King Arthur (ca. A.D. 500), during the Vendel era (550-793) as the tribe most closely associated with that great Pendragon, these Devon Domnonii established a dual kingdom which included Armorica and the north coast of Brittany (Domnonie), from whose royal house eventually sprang the House of Stewart (which house inherited the crown of the Scots in 1371 and that of England in 1603).

The Stewarts are covered under the chapter on the Normans, having come to Scotland in the wake of Norman conquest of England, in which they served as allies of the dukes of Normandy. In Ireland the Dumnonii were generally known as the Laigin, and originally became overlords in the southeastern and central regions, and in Connacht. From there they later spread to other parts of Gaeldom.

Dumnonii (Britain)

The Cianacht encompassed the O’Connors (O Conchobhair) of Keenaght, and the Luighne. The O’Connors were lords of Keenaght, County Derry, until dispossessed by the O’Kanes shortly before the Anglo-Norman invasion in the twelfth century. The Luighne were of County Sligo, where they had settled as fighting men to the Northern Gaels in the early centuries A.D. The Cianacht were closely related to the Dealbhna and Saithne.

The Luighne or "race of Lugh," included the families of O’Hara (O hEaghra) and O’Gara (O Gadhra). The O’Haras descend from Eaghra, Lord of Luighne (now the Barony of Leyney) in South Sligo, who died in 926. In the fourteenth century the O’Haras divided into two branches, the heads of which were known as O’Hara Boy (Buidhe), the Yellow O’Hara, and O’Hara Reagh (Riabhach), the Speckled or Brindled O’Hara. A branch of the family settled early in County Antrim, and became very important there.

The O’Garas were once one clan with the O’Haras, and together their territory, Luighne, included the modern baronies of Corran and Leyney in South Sligo, and Gallen and North Costello (Sliabh Lugha) in Mayo. About the end of the tenth century the two families separated, and divided the territory between them, the O’Garas taking the Mayo portion. They were driven from their territory by the Jordans, Costellos and other Anglo-Norman settlers, and resettled in Greagraidhe, in Sligo, now the Barony of Coolavin, and were later known as lords of Coolavin. They built their stronghold, Moygara, at the northeastern extremity of Lough Gara. Branches went to Munster before the end of the sixteenth century, and are known as Geary or Guiry. The O’Duanys or Devanys of Sligo are a branch of the O’Garas.

The Dealbhna Eathra and Dealbhna Nuadat were closely related to the Cianacht and Saithne. They originally comprised a single tribal kingdom in the Roscommon-Offaly area, but in course of time the various branches of the Deal bhna became separated under different overlordships, just as the Uí Maine became separated from their collateral kinsmen to the northeast of the Shannon, the Oirghialla, by the growing apart of the North Gaels which itself resulted in the ultimate overkingdoms of the Connachta and Uí Neill. The Dealbhna Eathra were situated to the east of the Shannon around Clonmacnoise, as a semiindependent tribal kingdom nominally subject to the Southern Uí Niell. Their chief families in medieval times were the MacCoghlans and O’Conrahys.

The Deal bhna Nuadat were centered on the other side of the Shannon, between it and the River Suck in County Roscommon, and were tributary to the Uí Maine. Their later representatives are the O’Hanlys of Connacht. The O’Hanlys (O hAinle) were chiefs of Cenél Dobhtha, called in later tirnes Tuaohanly and Doohy Hanly, being a district along the River Shannon north of Lough Ree. The O’Hanleys held this territory as late as the seventeenth century as tributaries of the O’Connor Don of Uí Maine. In the late sixteenth century several related gentlemen of the name were given in succession the office of "Seneschal" ("Royal Officer") of "Tohahohanly" under Queen Elizabeth I.

The Saithne were closely related to the Cianacht and Dealbhna. They originally inhabited a territory in the southern part of the kingdom of Brega, the kingship of which they in ancient times had shared with kindred groups. Their lands in Brega lay southeasterly, midway between the River Boyne and the River Liffey. Their later representatives were the O’Caseys. The O’Caseys (O Cathasaigh) were originally lords of Saithne, in the north of the present County Dublin, until they were dispossessed by the Normans under Sir Hugh de Lacy soon after the Anglo-Norman invasion (twelfth cenwry). Afterwards they became an important Erenagh (church) family, being hereditary keepers of Kilarduff and Dunfeeny in County Mayo, Cloondara and Tisrara in County Roscommon, and Devinish in County Fermanagh.

The Ciarraighe Locha na nAirne were originally part of a greater kingdom, the tribal kingdom of Ciarraighe, centered at Cruachu (the ancient capital of Connacht). This kingdom was fragmented by the Uí Briuin of the North Gaels during the late eighth century or early ninth century. They may have been, in more ancient times, closely related to the ancestors of the Oirghialla, the allies of the North Gaels (in the Heroic Age tales of the North, the "Ulster Cycle," Cruachu is the center of the Gaelic-Laiginian alliance). The Ciarraighe were indigenous to Connacht. Their main representatives in the Middle Ages were the O’Kierans (O Ceirin) of northwest County Mayo. The native territory of the O’Kierans was in the south of the barony of Costello, but they were reduced in power there by the Anglo-Norman encroachment, and branches in Donegal and Clare became more important.

The Ciarraighe Luachra were the original tribe of North Kerry, a branch of the Ciarraighe. Before the Anglo-Norman invasion had had a semiindependent kingdom between Tralee and the Shannon. Their chief family was that of O’Connor (O Conchobhair) of Kerry, whose stronghold was at Carrigafoyle, near Ballylongford. They held the Barony of lraghticonor in the extreme north of County Kerry after the southern part of their territory was encroached upon by the Fitzmaurices of Clanmaurice and other Norman settlers. The O’Connors held lraghticonor down to the reign of Elizabeth, when it was confiscated by the English and given to Trinity College.

The Eile were originally a tribe of western King’s County (Offaly), where place-names recall their early residence in that region. After the battle of Druim Derge (A.D. 516), at which battle they were decisively defeated by the expanding southern Uí Neill, they migrated to the area known after them as "Ely" in the south of Offaly and including northeast Tipperary. Their chief families in later times were the O’Carrolls of Ely, the O’Mahers, the O’Riordans and the O’Flanagans. The O’Carrolls (O Cearbhaill) descend from Cearbhaill, Lord of Ely, who was one of the leaders at the famous battle of Clontarf in 1014. The head of the O’Carrolls was originally lord of all Ely, but after the Anglo-Norman invasion their power was restricted to South Offaly, which was subsequently called Ely O’Carroll.

The Uí Failghe, closely related to the Eile, had probably separated from them by A.D. 516, the yeare of the defeat of the Eile at Druim Derge by the Southern Uí Niell. The Uí Failge descend from Failge Berraide, who a few years earlier had won the battle of Fremainn Mide (A.D. 510). This victory probably accounts for their being able to remain in the more northerly portion of Offaly while their cousins, the Eile, were forced to migrate south. The chief families of the Uí Failghe include the O’Connors of Offaly, the O’Mooneys, MacColgans, O’Hennesseys, O’Holohans, O’Dempseys and O’Dunnes.

The O’Connors (O Conchobhair) of Offaly were a powerful and warlike sept of the northeast of what is now County Offaly. They descend from Conchobhar, son of Fionn, Lord of Offaly, who died in A.D. 979. From their stronghold at Dangan, now Philipstown, they successfully defended their territory from the English of the Pale (i.e. County Dublin) for more than 300 years. They were finally dispossessed by the English about 1550. The O’Mooneys (O Maonaigh) of around Ballymooney in County Offaly are a branch of the O’Connors.

The Clann Cholgan included the families of MacColgan, O’Hennessy and O’Holohan. The MacColgans (Mac Colgan) were chiefs of the territory around Kilcolgan in the extreme northeast of County Offaly. The O’Hennessys (O hAonghusa) shared the lordship of Clann Cholgan (i.e., their clan-name was applied to the territory they possessed) with their kinsmen the O’Holohans (O hUallachain). Their territory comprised the present barony of Lower Philipstown, a district adjoining the hill of Croghan, near Kilbeggan, and lying just east of the O’Connors in northeast Offaly. A branch of the O’Hennessys were chiefs of Gailenga Beg, the district between Dublin and Tara, until they were dispersed into Offaly as a result of the Anglo-Norman invasion. Some of the O’Hennessys spread early into Tipperary and Clare. In County Clare they are now known as Henchy or Hensey.

The Uí Riagain or O’Dunnes (O Duinn) were chiefs of Uí Riagain in the northwestern corner of County Leix. They were, along with their kinsmen the O’Connors and O’Dempseys, one of the chief families of Leinster. A branch of the family possessed a territory around Tara until dispersed about the same time as the O’Hennesseys of that area (see above). The clan-name Uí Riagain, Anglicized Iregan, may reflect some relation to the sept of O’Regan (O Riagain) of the Southern Uí Neill, one of the Tribes of Tara, which settled in Leix after the Anglo-Norman invasion.