STRONG CHECKS/COUNTERS
The set I've come to use the most in safely dealing with Swirlix is
Hazekrow. At worst, you will trade one for one when dealing with Swirlix use Hazekrow. After Hazing, nothing Swirlix throws at you will OHKO, and after wards you can Thunderwave safely or switch to something else if it's Belly Drum. Unboosted and paralyzed Swirlix is basically a piece of shit. This set's main use it beating Cotton Guard + Calm Mind Swirlix. Draining Kiss barely does 56%, which you can Roost off easily while waiting for paralysis.
The second best way is
Paraflinch Slowpoke. Slowpoke can survive +6 Play Rough as well as +2 Dazzling Gleam with ease. Paralysis and Zen Headbutt can easily chunk it for a good amount with a 38% chance to paralyze or flinch. While you don't always beat Swirlix 1 vs 1, at worst you will trade 1 for 1.
Foongus is an obvious one... But TBH it's not 100% reliable. If you run Clear Smog you lose 1 vs 1 to Belly Drum, and if you run Sludge Bomb you lose to Calm Mind. But it's a decent check.
Grimer is the last counter/check that is decently reliable. It can OHKO with Gunk Shot and still even pose a threat to +6 def due to it's resistance to Draining Kiss' healing capabilities and strong poison stab. It can survive any boosted Swirlix move, and would probably only lose to some retarded Psychic + CG set which is shit.
WEAK CHECKS
Already mentioned before, but Prankster
Cottonee and
Purrloin (trash is irrelevant) is a decent way to deal with Cottonee. You can swap it in if it tries to set up, forcing it out. If they actually attack, then you can Giga Drain, putting them in KO range of other moves without activating Berry Juice. Obviously a very flimsy check, but with the right mindgames played, it can be effective.
Magnemite on paper looks decent, but in practice can only check Belly Drum Swirlix. Magnemite cannot OHKO a +1 sdef Swirlix, and Swirlix can break Sturdy with a Fairy move and then OHKO the next turn with Flamethrower. If you have CM Swirlix covered then Magnemites a great candidate for dealing with BD.
Aron? It beats most Swirlix sets (Roar for CG)... But it's not too terrible viable when facing teams without Swirlix. It's OK I guess, but there are dozens of things I'd use before Aron. The same thing applies for
Koffing and Wooper, it works but it's not that great overall outside of dealing with Swirly.
REVENGERS
Life Orb Meditite is a decent revenger of the BD and Calm Mind set. Unfortunately many players like myself have taken up the habit of using either Cotton Guard or Protect on their Swirlix sets, rendering Meditite fairy ineffective.
Fletchling's no item Acrobatics is amazingly powerful, being the strongest priority move in the metagame outside of things like Life Orb/Banded Sucker Punches. Fletchling can easily revenge Swirlix from around 70%. It's only a revenger and not a counter or a reliable check though, being unable to switch in on Swirlix setting up.
Banded Bunnelby's Quick Attack is in the same category of strength as Fletchling's Acrobatics as well.