The thing is that the Irish word tiarna is pronounced TEER-nuh, not tee-AHN-uh. It wouldn’t make any linguistic sense for Tiarne (tee-AHN) to come from a masculine Irish word that is pronounced completely differently, rather than just being an alternative spelling of a nearly identical, fairly popular girls’ name.
https://nanowrimo.org/participants/christine-seaforth-finch
http://christineseaforthfinch.blogspot.com/