Examples:
cartography
card
archaeology
archetype
monarchy
legal
loyal
purse
personal
cinema
kinase
car
carpenter
hippopotamus
Mesopotamia
dative
data
date
...
Middle Irish
techt
←
Proto-Celtic
*tixtā
(inherited)
←
Proto-Indo-European
*steygʰ-
(root)
←
Middle Irish
techtaid
(back-formation)
←
Old Irish
techt
(inherited)