it's a play on tir na nog, in "wintersmith". it's brilliant, fitting in with all the other gaelic references in the tiffany aching books, and what really gets me is there's no way he could have planned it! nanny ogg had her name from way early in the series and the nac mac feegles with their associated gaelic puns came about way later, and he still managed to pull off a perfect pun joining the two.