If driving an Arduino digital input pin, you need a resistor to define or pull up (to 5V or hi level) the "floating" node when the beam is broken. The eBay relay modules effectively have such a resistor on its input trigger pins.
Presumably somewhere in your Arduino sketch you define the digital pin connected to the IR sensor to be an input. If using pin number X, look for something like:
pinMode (x, INPUT)
Instead, try:
pinMode (x, INPUT_PULLUP)
and, lo and behold, you get:
Since your transistor-based circuit is already hooked up, just change the pinMode instruction and you should be off to the races. 
