course: Gramadach: An Tuiseal Ginideach - An Triú Díochlaonadh

The third declension has a mix of masc / feminine nouns, you can see which gender they are by the notes above. Masculine nouns have 'an + séimhiú, tS' in the genitive, feminine have 'na' before no changes. To change the end of the noun in this declension, the rule is: broaden +a, as you see in all the examples above.