I've come across two or three genealogy sites which detail a William McCathie who was apparently killed in action at Isandlwana. Further information:
Born 16 Oct 1835, Gateside, Strathmiglo, Fife, Scotland
Father - John, born 1 April 1803, Strathmiglo
Mother - Janet (nee Fernie), born c.October 1805, Strathmiglo
William married Helena Dorothy Foord (born 1846) 7 May 1866 in York, Natal and they had seven children between 1867 & 1877.
There are a number of spelling variations used in relation to the surname on the websites including McAthie/McCathie/MacCathie/McCathie but as far as I can see none of these appear in the casualty rolls for Isandlwana (happy to be corrected!) - therefore, the only conclusion I can come to is that this is the individual listed in England's Sons & For God, Queen & Colony as Sgt W. McCarthy 1st Btn 3rd Regt NNC (McCarty in Dutton's/Tavender's medal/casualty rolls respectively).
As ever, if anyone has any further information which would help to confirm this particular casualty's identity I'd be interested to hear.