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