The Trait: Nirnhoned is not showing up as a Blacksmith or Wood-Working craft able Trait, and yes the Trait Nirnhoned abilities have been previously researched.
There are filters at the top of the crafting screen that let you filter the gear levels, styles, and traits by which ones you're able to craft and which ones you have the proper mats for. If you know the Nirnhoned trait but don't have the necessary type of Nirncrux (Fortified for armor, Potent for weapons) then you won't be able to see the trait if those filters prevent it.
Similarly if you have learned it on some items but not all, and you're looking for the trait on an item where you have not in fact learned it for that item type. Learning it for 2-handed swords doesn't mean you can craft a 1-handed nirnhoned sword.