As far as I can tell, Nirnroots only appeared in Oblivion and Skyrim (and will be present in The Elder Scrolls Online too). In both games the amount you collect is tracked under the achievements page, there are quests associated with them and they can be used for alchemy too. The Elder Scrolls is set on the planet of Nirn, and so this plant was named after the planet.
They are generally found near water and emit a chiming noise, they can also be found in npc houses as potted plants. The chiming sound is sort of nice but it does produce a strong Pavlovian response to immediately pick it that I haven’t managed to shake yet… So in that sense it is highly irritating 
I am not alone…
I think my obsession with collecting Nirnroots came from the Oblivion quest, where you had to collect 100 of them. It was a quest that I remember being really hard as they seemed to be so rare. I still grin when I find one as it feels like an achievement, though I guess it’s not as I’ve since found out that there are 306 in game, (so I’m not sure why I remember this being so rare in Oblivion) and they are plentiful (and regrow) in Skyrim.
Oblivion Nirnroot quest
Skyrim Nirnroot quest
Skyrim Crimson Nirnroot quest
Oblivion Alchemy effects
Skyrim Alchemy effects
