We only add the h for one and two: aon chàr, dà chàr, trì càraichean. Obviously the Gàidhlig think that two does not deserve a proper plural but still mark it a little bit. Dà especially causes lenition, as in dà uair dheug - 12 o' clock. Or tha mi ag iarraidh dhà (it lenites itself because there is nothing else it could lenite). Maybe there is a explanation dating back to times when Gàidhlig developed but I don't know. You will get used to it. It belongs to the totally crazy fun stuff that makes the language so exciting.