This isn't the genitive. It's de, which causes lenition, and becomes d' before a vowel sound. de is generally used instead of the genitive when making a selection from several things (two of the balls, for example). Feabhra is lenited because of the preceding de. It’s a partitive dative (in a nominative form) rather than a partitive genitive, but since the nominative and genitive forms are identical, the reason is the de rather than which case Feabhra has.