Cahir or Caher?S0524 : Cahir CastleS0524 : Caher castleIrish
An Chathair, "the City (or enclosed fort)". Caher is the older spelling, used by the Ordnance Survey, but many modern authorities including the National Monuments Service and Google maps go for Cahir. The gazetteer lookup finds both English spellings, but displays only the Irish.
There are three other instances of the place-name Caher (in Counties Clare, Cork and Galway). A gazetteer search on Caher will find all four, but searching on Cahir turns up only this one (Tipperary).