As the name indicates, irradiated whiskey is simply whiskey which has absorbed radiation. Its basic effects are almost identical to those of standard whiskey, except that it also provides 10 rads when consumed.
A fourth bottle may be found inside a refrigerator inside Hell's Motel. This fridge is set to contain four of a random selection of irradiated foods, including irradiated whiskey. There is only a 29% chance of any specific food item appearing in that refrigerator, including irradiated whiskey.