In the complex ion $[Fe(CN)_6]^{3-}$, the ligand is cyanide ($CN^-$), which is named 'cyanido' in IUPAC nomenclature. Since there are six such ligands, the prefix 'hexa' is used. The overall complex is an anion (indicated by the negative charge), so the name of the central metal iron changes to its Latin root ending with '-ate', making it 'ferrate'. Let the oxidation state of Fe be $x$. We calculate the oxidation state as: $x + 6(-1) = -3 \Rightarrow x = +3$. Therefore, the complete IUPAC name is hexacyanidoferrate(III) ion.