A special situation occurs when two (or more) nouns in the genitive directly follow another noun. In these situations, Irish syntax often avoids a “double genitive” and only the second of the two genitive nouns takes the genitive form (or spelling). (In instances of more than two nouns in the genitive following another noun, only the final genitive noun takes the genitive form.)
Syntactically, though, both nouns are in the genitive. Rather than taking the usual genitive form (or spelling), the first of two nouns in the genitive is spelled according to its nominative form and lenited (when possible). This noun is often referred to as a “functional genitive” because, in spite of the form it takes, it functions as a genitive. The double genitive is always avoided when the second of two nouns in the genitive is definite.
Examples of functional genitives:
Murchadh mhac Briain (“Brian Ó Ruairc,” quatrain 2) [lenited nominative form (spelling) mhac used in place of genitive form in order to avoid double genitive]
go teach Fheidhlimidh mhic Daill (Keating, “Marbhadh,” paragraph 2)
The double genitive is not always avoided, however. In certain situations Irish syntax allows a double genitive if the second of two genitive nouns is indefinite. Similarly, when the first of two genitive nouns is a verbal noun, both nouns take the genitive form (or spelling).