Heat 2 tablespoons oil in a small skillet over medium-high heat. Add sliced jalapenos; cook and stir until browned and crispy, 3 to 5 minutes. Transfer to a small plate with a slotted spoon.
Heat 2 tablespoons oil in a large saucepan over medium-high heat. Put 4 popcorn kernels into the oil and cover until popped; add remaining kernels to the saucepan in an even layer. Cover and remove from heat, about 30 seconds. Return to heat and gently shake pan back and forth over the burner until popping slows to several seconds between pops, 1 to 2 minutes. Remove from heat and transfer to a large bowl.
Pour melted butter evenly over popcorn. Lightly sprinkle ranch dressing mix over popcorn; mix to combine. Crumble jalapeno slices and mix in.