Need help with IR Transmitter circuit
I know it's acting as a current limiter to the LEDs but confused about its role in feedback.

No, it's not acting as a current limiter. That's why you're confused.
R77 is part of a current controller. Opamp U8A operates to keep the voltage across R77 equal to the voltage on its non-inverting input. It will drive Q4 gate to whatever voltage is needed to maintain its two inputs at the same voltage. With a defined voltage across R77, there will be a defined current through it.
The input signal and the components around Q7 modulate the opamp's input voltage to modulate the controlled current.