From Ó Cathasaigh, ‘vigilant, watchful’ — distinct septs near Dublin and Meath, and separately in Cork and Limerick.
Gaelic Origin · Root meaning ‘vigilant, watchful’
Casey comes from cathasach, ‘vigilant’ or ‘watchful’, most likely a warrior's byname. Unrelated Ó Cathasaigh septs held ground both in the Dublin-Meath borderlands and, separately, further south in Cork and Limerick.
A name for a quality rather than a place — which may be exactly why so many unconnected families, each keeping their own watch, ended up sharing it.
Irish form: Ó Cathasaigh
