Ask Rengar Pridestalker, can I react to my own Irresistable Fae Folk pulling a lone enemy from the battlefield and go to that battlefield before they move and conquer?

Yes — you can react to your own Irresistible Faefolk trigger by playing a unit to that battlefield before the enemy moves there.

Here's how the timing works:

1. You move Faefolk to an empty battlefield → you become the attacker (you applied Contested).
2. Faefolk's "When I move…" trigger goes on the Chain → the game enters a Closed State.
3. In that Closed State, you (the controller of the newest Chain item) have priority and can play Reaction-speed cards/abilities (FAQ #11143).
4. You play Rengar, Pouncing (or another unit with Reaction) to that battlefield — Rengar's text "I can be played to a battlefield you're attacking" overrides the normal Base-only restriction (FAQ #8779). Rengar enters the battlefield during the Reaction window.
5. Rengar Pridestalker's legend ability ("When you play a unit, give a unit +1 [S] this turn") also triggers and goes on the Chain — you give the +1[S] to whichever unit you want before combat.
6. Faefolk's original trigger then resolves, moving the enemy unit to that battlefield.
7. You now have Rengar (+ Faefolk) vs the pulled enemy → Combat Showdown begins.

So yes: your Faefolk trigger creates the Reaction window, and you can absolutely jump in with Rengar before the enemy arrives.