A "tiarna" (Irish), or "tighearna" (Scottish), both from the Old Irish "tigerna", is a lord in the Gaelic world and languages.
Tierney is a modern version of the surname of "Tiarna", even though spelt differently the meaning remains the same.
Sources:
https://en.m.wikipedia.org/wiki/Tigerna
https://en.m.wiktionary.org/wiki/Tiarna
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