What's the proper way to connect this IR Sensor Module to a GPIO input? I'm guessing it's output is 5v, not 3.3v that the GPIO wants.
Given that the device is sold as an Arduino module it probably does output at 5V.
A level shifter of some sort would be best to convert the 5V output to Pi safe 3V3.
I tend to use a couple of resistors as a voltage divider.
A couple of resistors with one twice the value of the other would be almost perfect giving a voltage of 3.333V from 5V. At a pinch you could use a pair of resistors with the same value as that would give 2.5V which will likely still be seen as high at the Pi.
A level shifter is the safe option. They are not expensive. If you are just looking to drive a logic input a voltage divider should also work.
I am able to drive a (5v) relay with the OUT line.
I don't see anything on the board that looks like it would drive a relay (unless you are talking about a relay module with logic input). Directly connecting a relay risks damaging the device.