Study these examples:
* chaith sé é féin ar ghealacán a dhá ghlún: he himself fell on the caps of his two knees (he fell down on his knees) [Mo Sgéal Féin, Peadar Ua Laoghaire]
* do maithfí obair dhá bhlian dó: he was let off two years’ work [Mo Sgéal Féin, Peadar Ua Laoghaire]
*brí agus bunús an dá leitir: the meaning and essence of the two letters [Niamh, Peadar Ua Laoghaire]
Two knees, requiring the dual, would normally be dhá ghlúin. as found elsewhere in Mo Sgéal Féin—tharaing sé chuige mé go rabhas idir a dhá ghlúin aige: he pulled me towards him till I was between his two knees. But the genitive dual is different in that it normally requires use of the genitive plural (often identical to the nominative singular).
The example above of ar ghealacán a dhá ghlún illustrates the genitive dual. Originally the nominative singular was glún, and the genitive plural was identical to this, although PUL is on record as saying he only ever heard glúin in the nominative singular (reflecting a process whereby the nominative of this word became aligned with the dative), so that the genitive dual is broadened compared with the nominative singular. The example of dhá bhlian is interesting too. This is also a genitive plural that is a broadened version of the nominative singular bhliain (see also súil-súl).
Consonant mutations on the noun following dhá are determined by the possessive particle preceding dhá: os cionn a dhá shúl, over his two eyes; os cionn a dhá súl, over her two eyes.