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]
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, and in this case it is identical to the nominative singular, glún. The example of dhá bhlian is interesting too. This is also a genitive plural, but in the case of this noun, the genitive plural is a broadened version of the nominative singular bhliain (see also súil-súl).