[From Manx Soc vol 2, Kelly's Manx Grammar]
THE CONSTRUCTION OF SUBSTANTIVES.
When two substantives come together belonging to divers things, the latter, if it be masculine, and the article y or yn precede it, shall change its initial into its soft: as folt y ching, the hair of the head; duillag y villey, the leaf of the tree: but words beginning with d, j, t, of the mutable consonants, are not subject to this change: as kione y jalloo, the head of the image; mac y Jee, the son of the God; ben y dooinney, the man's wife; ben y thie, the woman of the house.
When two substantives come together, if the latter be of the feminine gender, the article ny, not yn, is used in the genitive, and the mutable consonant remains unaspirated: as cass, a foot, yn chass, the foot, boyn ny coshey, the heel of the foot; sooill, an eye, yn tooill, the eye, clagh ny sooilley, the apple of the eye.
If the latter substantive be the proper name of a country, town, or place, without an article, the latter changeth its radical initial into its soft: as Ellan Vannin, the Island of Mann; mac Yee, the son of God; thie Ghavid, the house of David.
Both substantives being common, the latter is determined by the gender of the former: as (fem.) slat hoost; (fem.) clagh wyllin; (fem.) feil1 vuc, swine's flesh; stroïn (masc.) muc, a swine's snout; cloan (fem.) ghooinney, a man's children; mac (masc.) dooinney, a man's son; ben ghuilley; mac ben. But if the former substantive be of the plural number, then the latter is immediately subjoined with its radical initial: as slattyn soost, flails; claghyn mwillin, mill-stones.