Who ruled Ireland before the Normans. The Déise, Osraige, Thomond, the Uí Néill, the Eóganachta and twenty more kingdoms and peoples, where they were in today's counties, and the surnames that came out of them.
Before the counties there were túatha: small kingdoms, a hundred and more of them, grouped under over kings who spent most of a thousand years rearranging each other. This is the layer that explains why a surname belongs to a place.
The south
Na hEóganachta (The Eóganachta): The royal kindred of Munster, who held Cashel for four hundred years and then lost it to a people they had treated as vassals.
Dál gCais (The Dalcassians): The people who came from nowhere in the tenth century, took Munster off the Eóganachta, and produced the one high king everybody can name.
Tuadhmhumhain (Thomond): North Munster. The Ó Briain kingdom that outlived the high kingship by four hundred years.
Deasmhumhain (Desmond): South Munster. Mac Cárthaigh country, and the name of the earldom that tried to break away from the Tudors and was destroyed for it.
Urmhumhain (Ormond): East Munster. The third of the three Munsters, and the one that became a Butler title rather than a Gaelic kingdom.
Na Déise (The Déise): Waterford and south Tipperary. A people named for their rank rather than their blood, and the only one of these names still shouted at a hurling match.
Éile (Ely): The Offaly and Tipperary border. A small kingdom with one very large family in it.
The southeast
Laigin (The Laigin): The southeast. The people who gave Leinster its name, and whose last king invited the Normans in.
Osraige (Ossory): Kilkenny and west Laois. A kingdom that spent a thousand years being fought over by Munster and Leinster, and outlasted both.
Uí Failge (Offaly): The Offaly midlands. Bog and water made it hard to take, and it stayed Gaelic longer than its neighbours for that reason.
Loígis (Laois): County Laois. A people planted there to hold a frontier, who were still holding it a thousand years later.
The north
Uí Néill (The Uí Néill): The most successful family in early Ireland, north and south, and the reason Tara means what it means.
Cenél nEógain (Tyrone): The Ó Néill country. Tír Eoghain, Eógan's land, which is still the name of the county.
Cenél Conaill (Tirconnell): Donegal. Columba's people, and later the Ó Domhnaill who went into exile with the Ó Néill.
Airgialla (Oriel): Mid Ulster. Not one people but a group of smaller ones, named for what they owed rather than who they were.
Ulaid (The Ulaid): The people who gave Ulster its name, and then lost most of Ulster.
Dál Riata (Dalriada): North Antrim and Argyll. The kingdom that straddled the sea, and the reason Scotland speaks a Gaelic language.
The west and the middle
Connachta (The Connachta): The west. The province is named for them, and the last two high kings of Ireland came out of them.
Uí Maine (Hy Many): East Galway and south Roscommon. A thousand square miles of Connacht under one family for five hundred years.
Uí Fiachrach (Hy Fiachrach): The Moy estuary, Sligo and north Mayo. One of the three great Connacht kindreds, and the one that lost the province.
Bréifne (Breifne): Cavan and Leitrim. One kingdom that split into two along a line that is still the county border.
Anghaile (Annaly): County Longford. A small midland kingdom that is more or less exactly one modern county.
Mide (Meath): The middle kingdom, and the fifth province. Tara is in it, which is the whole point of it.
A word about borders
None of these had edges you could draw. A kingdom was the people who owed you tribute, and that changed with every generation and every war, so the counties given here are a rough modern equivalent and nothing more. Where the popular story of a people turns out to be a medieval invention, the page says so.
For the legendary peoples, the Tuatha Dé Danann and the Milesians, see mythology: that is a different kind of history and it is kept separate on purpose.