The Dindṡenchus often gives fanciful etymologies for names of places. This is not to say that we can do better in every case; sometimes we are also at a loss. Although most placenames can be etymologised, Ireland has obscure onomastic elements, as well , e.g. OIr. Temair “Tara”. https://twitter.com/dindshenchas/status/1252303191292760067

@logainm_ie records 10 such places across the island ( https://www.logainm.ie/ga/s?txt=teamhair&str=on). Hogan’s Onomasticon Goidelicum has 22 different Temairs ( https://www.dias.ie/celt/celt-publications-2/onomasticon-goedelicum/). Medieval glossators explained temair as a word for “any high place, eminence, hill” ( http://dil.ie/40480 ),...

...which looks reasonable, but it lacks a good etymology. It has been suggested to connect it with the PIE root *temh₁- “to cut, separate” < *temh₁ri- (cf. Gr. τέμενος “separated piece of land, precinct, holy area” < *temh₁no-), but that should result in OIr. **tamair,...

...not temair. The Latinised form of the name Temoria may indicate an o in the second syllable, but that doesn’t lead to a morphologically transparent preform, either. Perhaps a borrowing from a pre-Celtic language of Ireland.