I believe in 8-bit REC709 0IRE corresponds to 16 reference black because actual 0-15 is used for sync and undershoot. Similarly 100IRE would correspond to 235 (with 236-255 being overshoot and possibly specular highlights), but that material in both the extreme highs and lows would most likely not actually be represented in actual picture material. Maybe John Mason or someone could chime in to verify this.
As far as NTSC goes it depends whether you are talking about Rec601 digital or analog. In analog NTSC (USA only not Japan) color black is 7.5IRE with "Super Black" being 0, but in digital REC601 Standard Definition, the values are still 16 - 235. This however causes ALL KINDS of production issues when converting from standard definition analog to digital as many cheaper digital formats like MiniDV incorrectly will read value 16 as 0IRE instead of offsetting it to 7.5IRE.
It begins to get really complicated when you're dealing with material shot on film and scanned in LOG space and then converting it to REC709, REC601, Digital Cinema (DCIP3) spec (or just working with it on a computer in RGB space, or trying to properly display it in a DI facility) correctly can get really complex quickly to figure out.