Well, although it seems weak to CB Terrakion on paper, I think it would be manageable simply because of Tentacruel + Quagsire - Terrakion isn't OHKOing Tentacruel - Jolly CB does 71.2% - 83.8% with SE to Tentacruel, meaning that you are left with 36% on average and then Protecting the next turn leaves you with 48%, meaning that you are able to switch to Quagsire on the Stone Edge and then later heal Tentacruel with either Protect or Wish.
Secondly, the sheer amount of Pokemon with Protect on this team means that CB Terrakion (which is the most dangerous variant of Terrakion for this team to handle) can only damage the team if it comes in safely on Dragonite, Quagsire, or Ferrothorn; otherwise, its move will be predicted and easily switched into.
Dragonite using Roost would be the easiest to switch into (Quaggy and Ferro both can OHKO Terrakion), but even then, Dragonite has a chance to beat CB Terrakion one on one - it's forced to use SE to win, and Dragonite can take one SE (assuming full health) and use Hurricane, weakening Terrakion immensely, before switching to Quaggy.
tl;dr - Terrakion is killing one Pokemon at most, although the most likely scenario is that M Dragon is forced to let a Pokemon be weakened to ~30%.
Originally Posted by Fat Eggbert
Also, just curious, where does the name IR come from?