Pretty simple actually, and you won't even need a PIC microcontroller (though some will try to sell you on it, be warned...).
First, there is a cheap and easy to find IR reciever module (costs arond $1.00, probably around $5.00 at Radio Shaft). It's the same module found in any VCR, TV or and thing with a remote actually, pretty common. Salvaging one from a dead VCR is one way, but buy one you can get the pinout. Generally, the center pin is the signal out, and they run off 5 VDC. When it sees an infrared signal at or around its center frequency (around 38 khz), you get an output, which you can use to turn on/off your LEDs.
Can't give you a circuit diagram at this point, because I don't know if you want the LEDs to stay ON continously until you want to turn them off, or just on while you hold down the button. I don't know the specs on you light bar, voltage/current draw, so not sure what would be required to do the switching.
Also, any IR remote you have will likely trigger this, so will you need this to be specific? I mean, if your light bar is in the TV room, changing channels or something could toggle the lights...
Need to add a few details on what you have planned, there are dozen of people here who could throw this together in minutes, if given enough information to get them started. Be as specific as posiable.