Declension of Irish Names
The declension of names follows the ordinary rules for common nouns. The following points may, however, be noted:—
1. Many names belong to more than one declension.
2. A few names have in the course of time changed their declension.
3. Names of the fifth declension which form their genitive case by adding ch or n, generally drop these letters in the spoken language.
4. Diminutives in -óg are masculine and invariable.
5. In the case of compound names, the part to be inflected depends on the nature of the compound. Sometimes both parts are inflected, as Cú dubh, gen. Conduibh; sometimes the second only, as Dubhchú, gen. Dubhchon. When the second part is already in the genitive case the first alone changes, as Cú Uladh, gen. Con Uladh, Conuladh.
6. The nominative case is sometimes used for the genitive, and vice versa.
A perusal of the list of surnames will show the genitive form of most names.