Declension of Irish Surnames

In surnames of Types I, II, III, IV, V, and VII, the name or designation of the ancestor is in the genitive case and remains unchanged in the declension of the surname, except that its initial letter is sometimes aspirated or eclipsed, if a consonant, or has h or n prefixed, if a vowel. With this exception Mac and Ó alone change.

Ó, or Ua, and Mac are thus declined:—

Ó, is the usual form in the nominative case, Uí in the genitive. When the name of the ancestor begins with a vowel, h is prefixed after Ó in the nominative case singular,* and n in the genitive plural. Ó sometimes aspirates f in the nominative case singular, and always eclipses in the gen. plural, if the name commences with an eclipsable consonant. Uí causes aspiration in the singular. Mac sometimes aspirates the initial consonant of the name in the nominative case singular. The M of Mac is itself frequently aspirated in the nom. case singular after the Christian name; always in the genitive case, and sometimes

EXAMPLES:

The plural forms of Ó and Mac are now met with only in place-names derived from clan or family names, as: Uí Cinnsealaigh, Uí Dróna, Cúil Ó bhFinn, Mainistir Ó dTórna, i nUibh Laoghaire. The modern collective plural of surnames of these types is formed by prefixing Muinntear, Clann, or Síol† to the genitive case of the name of the ancestor, as:

Muinntear Bhaoighill, the O'Boyles;
Muinntear Éilidhe, the O'Healys;
Muinntear Raghallaigh, the O'Reillys;
Muinntear Ruairc, the O'Rourkes;
Clann Amhlaoibh, the MacAuliffes;
Clann an Bháird, the MacWards;
Clann Dhiarmada, the MacDermotts;
Clann tSíthigh, the MacSheehys;
Síol mBriain, the O'Briens;
Síol gCeallaigh, the O'Kellys;
Síol Mórdha, the O'Mores;
Síol Uidhir, the Maguires.

Muinntear is used in the case of Ó-surnames; Clann, with a few exceptions, is confined to Mac-surnames.

Síol is now used only in literature. Muinntear and Clann are sometimes prefixed to the genitive case of the surname, as: Muinntear Uí Cheallacháin, the O'Callaghans; Clann Mhic Conmara, the MacNamaras. The following examples show the declension of surnames in -ach (Types X, XIV, XV):—

Surnames of Types VI, VIII, XI, XII are not declined. All these surnames form their collective plurals like surnames in -ach, as: na Baróidigh, the Barretts; teach na mBúrcach, the house of the Burkes.

Surnames of Type IX follow the rule of adjectives. They form their plural like surnames in -ach.

Surnames of Type XIII are invariable. The plural is formed by prefixing Muinntear.

In surnames of Type XVI., Mac alone changes.

A personal epithet, or cognomen, comes between the Christian name and the surname, and, if an adjective, agrees in case with the Christian name, as: Domhnall Bán Ó Briain, bó Dhomhnaill Bháin Uí Bhriain.

Sometimes the father's name, in the genitive case with Mac prefixed, is inserted in the same position to distinguish persons of the same name and surname, as: Domhnall mac Dhonnchadha Ó Briain; or the patronymic may follow the surname, as: Domhnall Ó Briain mac Dhonnchadha. Mac always aspirates in this case. The mac is now usually dropped, but the aspiration remains, as: Pádraig Thaidhg Óig Ó Conaill.

When two Christian names are used, the second is put in the genitive case, with its initial letter aspirated, —Mac being understood if the father's name, and giolla if the name of a saint, as: Seán Pheadair Ó Néill, John Peter O'Neill.‡

In the case of a double surname in English, as Patrick Sarsfield O'Donnell, Hugh O'Neill Flanagan, the first surname assumes the -ach termination, thus: Pádraig Sáirséalach Ó Domhnaill, Aodh Niallach Ó Flannagáin.

When a personal cognomen of the ancestor appears in the surname, it agrees in case with the name of the ancestor, as: Seán Mac Mhuiris Ruaidh, Tadhg Mac Conaill Óig.

An agnomen used to distinguish different branches of the same family agrees in case with the surname; in other words, it is in the same case as Mac or Ó, as: Maghnur Mac Diarmada Ruadh, Ó Conchobhair Donn.

* There are a few exceptions.

† Muinntear and Clann cause aspiration, Síol eclipsis.

‡ This rule is not always observed by present-day Irish speakers.