co- comes from older comh meaning fellow, common, mutual similarly to English-from-Latin co- (from older Lat. con, from prep. cum) in co-operate (both from Proto-Indo-European *ḱóm ‘with’). So co-ogha is a fellow grandchild, co-grandchild (which makes sense, your cousins are other grandchildren of your grandparent).
The same co(mh)- is in còmhla (ri) together (acc. to MacBain’s from còmh-làmh, with fellow hand, hand in hand, hands together (with)).