Examples:
cartography
card
archaeology
archetype
monarchy
legal
loyal
purse
personal
cinema
kinase
car
carpenter
hippopotamus
Mesopotamia
dative
data
date
...
Middle Irish
oc
←
Proto-Celtic
*onkus
(inherited)
←
Middle Irish
ocus
(cognate)
←
Old Irish
oc
(inherited)
→
Scottish Gaelic
aig
(inherited)