Where the O’Dea surname comes from, the battle that made Dysert O’Dea famous, what DNA says about the family’s Dál gCais roots, and which test answers the question for your line.

The O’Deas (Ó Deághaidh, from Déaghaidh, a 10th-century chief) ruled a large part of the barony of Inchiquin, just north of Ennis in County Clare. Their centre was Dysert O’Dea, where their tower house and a round tower still stand beside a carved high cross. The name also appears as O’Day or Day, and it remains a well-known Clare surname.

O’Dea at a glance

  • Irish form: Ó Deághaidh
  • Meaning: descendant of Déaghaidh
  • Seat: Dysert O’Dea, near Corofin, County Clare
  • Kindred: Uí Fearmaic, counted among the Dál gCais septs
  • Variants: O’Day, Dea, Dee, Day
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Are you a “Real” O’Dea or a “Royal” O’Dea?
The O’Dea DNA project has found separate Dalcassian branches inside the surname, and one of them carries O’Brien DNA. A Y-DNA test shows which branch you belong to. Family Tree DNA is the only major testing company with dedicated Y-DNA tests, and it hosts the O’Dea/O’Day/Dee project run with the Dysert O’Dea Clan Association.
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Origin of the name

The clan’s own historian, Risteárd Ua Cróinín, traces the name to Déaghaidh, a chief mentioned in Keating’s history under the year 934, and shows him in the pedigrees as a descendant of Cas, the ancestor of the Dál gCais. Edward MacLysaght calls the O’Deas one of the principal Dalcassian septs. The family belonged to the Uí Fearmaic group along with the O’Quinns and O’Griffins, and some modern scholars question whether that group was Dalcassian by origin or was absorbed later. DNA has since given a clearer answer.

Dysert O’Dea and the battle of 1318

The O’Dea territory took in much of Inchiquin, and the townlands of Curragh O’Dea and Tully O’Dea still carry the name, as well as Dysert itself. The family were coarbs of the old monastic site at Dysert, managing its church lands, and they were long-standing allies of the O’Briens. An earlier chief, Flaherty O’Dea, fell beside them at the battle of Móin Mór in 1151.

The family’s great day came on 10 May 1318. Richard de Clare led a Norman army into Clare to settle the struggle for Thomond, and the O’Deas and their O’Brien allies, backed by the O’Hehirs and O’Conors, met it at Dysert. Conor O’Dea split the Norman force by drawing part of it across a ford, and de Clare was killed in the fighting. The victory ended Norman hopes of holding Thomond and kept Clare Gaelic for another two centuries.

Later records are more domestic. In 1568 Bishop Conor O’Dea of Kilmacduagh left the castle and banqueting hall of Dysert to his nephews, on condition that they kept the peace and let the whole household use “the great brewing pan”. In 1683 Michael O’Dea repaired the high cross, and the inscription recording it is still on the base.

O’Dea DNA

The O’Dea/O’Day/Dee project on Family Tree DNA has tested close to ninety men, and most O’Deas sit on Dalcassian branches marked by SNPs labelled R-DC. Inside that, the project sees at least two groups. The “Real O’Deas” may descend from a man who used the surname before 1318. The “Royal O’Deas” carry O’Brien DNA, which suggests a boy of O’Brien blood grew up as an O’Dea, perhaps through fosterage. Either way, both groups belong to Brian Boru’s kindred, and the Royal branch descends from his own line.

Separating the branches takes Y-DNA testing. A Y-37 test from Family Tree DNA confirms the R-L226 pattern, and Big Y-700 is the test that tells a Real O’Dea from a Royal one. If your O’Dea link is through a mother or grandmother, Family Tree DNA’s Family Finder is the right choice. Our DNA guide explains how to read the results.

O’Dea places to visit

  • Dysert O’Dea: the O’Dea tower house, the round tower, the Romanesque doorway of St Tola’s church and the high cross, all within a short walk, near Corofin.
  • Ennis: 15 minutes south, with the friary and the Clare County Library’s local studies centre. See places to stay around Quin and Ennis.
  • The Burren and the coast: Corofin is the southern gate to the Burren, and the Cliffs of Moher are under an hour away. Book a Cliffs of Moher tour.
  • Killaloe: the seat of the O’Brien kings the O’Deas fought beside. Plan it with our Brian Boru trail.

Related Dál gCais surnames

See the MacMahons and Considines, both from the O’Brien line, and the Clancys and McInerneys of Clare, or browse all Brian Boru kindred surnames.

Frequently asked questions

Is O’Dea an Irish name?

Yes. O’Dea is the anglicised form of Ó Deághaidh, a Gaelic family of County Clare. O’Day and Dee are versions of the same name.

Are the O’Deas related to Brian Boru?

Yes. DNA places most O’Deas on Dalcassian branches, and one group, the so-called Royal O’Deas, carries O’Brien DNA from Brian Boru’s own line.

What happened at the Battle of Dysert O’Dea?

On 10 May 1318 the O’Deas and their O’Brien allies defeated Richard de Clare’s Norman army at Dysert. De Clare was killed, and Thomond stayed under Gaelic rule.

Where is the O’Dea family from?

Dysert O’Dea in the barony of Inchiquin, County Clare, between Ennis and Corofin.

Sources

  1. Clare County Library, O’Dea, Ó Deághaidh, O’Day, O’Dee.
  2. Family Tree DNA, O’Dea/O’Day/Dee Project: background.
  3. Family Tree DNA, O’Dea/O’Day/Dee Project: results.
  4. Risteárd Ua Cróinín, O’Dea: Ua Déaghaidh, The Story of a Rebel Clan, Ballinakella Press (1992).