Where does “Callan” come from?

Callan derives from Irish Ó, a prefix meaning "descendant of," though the specific personal name it originally modified remains unattested.

Ancestry of “Callan”, step by step

Step	Language	Word	Meaning
1	Irish	Ó Cathaláin	—
2	Irish	Cathalán	little Cathal
3	Old Irish	Cathal	—
4	Proto-Celtic	Katuwalos	strong in battle
5	Proto-Celtic	katus	battle
6	Proto-Indo-European	kéh₃tus	fight
7	Proto-Indo-European	-tus	Derives action nouns from verb roots

Callan — Etymology, Origin & Word History | Etymology Explorer