Hi,
(1)if I remember correctly, the attach Interrupt with FALLING Didn't work for me at all. I used CHANGE(?) and then explicitly read the current digital pin value.
(2) did u check the electrical signal from the IR gate? For a digital input you need a digital signal, Ie the signal should be a square wave with steep slopes. In my case I needed a dumb amplifier using a single transistor to get such a clear signal.
HTH, Robert