I am starting to play a lot of top laners to get ready for my team when I stumbled upon Irelia. I just want to know if Mercury's Treads stack with her passive.
Tenacity stack multiplicatively with other forms of Crowd Control reduction.
Since everything that provides Tenacity right now gives 35 Tenacity, Irelia with any of the CC reduction items will have the following stats
Ionian Fervor stacks multiplicatively with tenacity: 1 champion: 41.5% 2 champions: 51.25% 3 champions: 61%
gbn's answer is correct. But I think it's helpful to show the details here.