Irish Gaelic and Welsh form of Katherine and its variants Katharine/Catherine/Catharine, from Greek "pure". ONC gives Caitlín as derived from the Old French form, Catheline, which is also the origin of Kathleen.

Two correspondents [AB & RB] have each noted an ancestor's sister, baptised Catherine in Scotland, known to the family as Kathleen and sometimes recorded as such in official records. Both Catherine and Kathleen are Anglicized forms of Caitlín and hence are classed here as "lesser synonyms" of each other as there is no direct connection between these names.  However because of these examples which are probably not unique, we have also classed these names as synonyms.

Another correspondent [LC] has found Caitlyn, a modern variant in spelling, in a family of Scottish ancestry, now in the US.