Interchange of Surnames

1. Some Irish families have two surnames, each derived from a different ancestor, or one derived from the name and another from a designation of the same ancestor, as:—

Ó Maolruanadha and Mac Diarmada,
Mac Conmara and Mac Síoda,
Mac Giolla Phádraig and Mac Séartha,
Mac Síomoinn and Mac an Ridire.

2. Some families of foreign origin have two Irish surnames, one an hibernicised form of the foreign name and the other a patronymic formed from the name or a designation of the ancestor, as:—

Condún and Mac Máigheóc,
Dalamara and Mac Hoireabáird,
de Búrc and Mac Uilliam,
Stondún and Mac an Mhíleadha.

3. Nearly a dozen families have two surnames one commencing with Ó and another with Mac, followed by the same ancestral name; but whether both surnames are derived from the same ancestor, or from two different ancestors of the same name, it is impossible to say. Examples:—

Ó Caocháin and Mac Caocháin,
Ó Codlatáin and Mac Codlatáin
Ó Geargáin and Mac Geargáin.

4. A few families have besides their surname a cognomen which is sometimes used instead, as:—

Ó Duinnshléibhe and Ultach.

All these double surnames were used interchangeably, so that the same person might be called indifferently by one or the other, irrespective of the anglicised form. In the majority of cases only one surname is now retained; but as the anglicised form is, in many instances, derived from the one that has become obselete, there is often apparently no connection between the anglicised form and its present Irish equivalent. Thus the surname Fitzpatrick is derived from Mac Giolla Phádraig, but the present Irish equivalent of Fitzpatrick in many parts of the South of Ireland is Mac Séartha or Mac Séathra, a new surname which the Fitzpatricks took from an ancestor named Geoffrey or Séartha Mac Giolla Phádraig. Similarly the Birminghams are called in Irish Mac Fheórais from an ancestor named Piers de Bermingham.

Besides the interchange of totally distinct surnames, our Irish name system admitted, with considerable latitude, of the substitution one for another of different forms of the same surname, and even of different surnames of the same or somewhat similar meaning. Hence we find the following classes and variants of surnames sometimes interchanged:—

1. Surnames of the same signification though differing in form, as:

Mac Carluir and Mac Cathail (Carlur and Cathal, each=Charles).

Mac an Mhadaidh and Ó Madaidhín (both from madadh, a dog).

2. A surname and its diminutive, as:

Mac Bruaideadha and Mac Bruaidín.
Ó Lachtna and Ó Lachtnáin.
Ó Scannail and Ó Scannláin.

3. Surnames derived from different diminutives of the same root, as:

Ó Branáin and Ó Branagáin,
Ó Ciaráin and Ó Ciaragáin,
Ó Dubháin and Ó Duibhín,
Ó Fiacháin and Ó Feichín.

4. Surnames derived from different genitive forms of the same name, as:

Ó Fearghusa and Ó Fearghuis.
Ó Fiaich and Ó Féich.
Mac an Bhreitheamhan and Mac an Bhreithimh.

5. Variants of the same surname owing to aspiration, attenuation, and interchange of letters, as:

Mac Domhnaill and Mac Dhomhnaill.
Ó Brolcháin and Ó Broileacháin.
Ó Dearáin and Ó Dioráin.
Ó Macháin and Ó Mocháin.

6. A standard or literary form and a corrupt or spoken form, as:

Ó Muirgheasáin and Ó Bríosáin.
Ó Caoindeal and Ó Caoinliobháin.
Ó hEidirsceóil and Ó Drisceóil.

7. An older form and a more modern one, as:—

Mac an Airchinnigh and Mac an Oirchinnigh.
Mag Aireachtaigh and Mag Oireachtaigh

A discrepancy (similar to that mentioned above) between the anglicised form and its present-day Irish equivalent often results from the interchange of these forms.