These sites are useful at checking Irish plates:

www.cartell.ie

www.mywheels.ie

If the plate doesn't show up (provided it's genuine), chances are the car was scrapped before 1993 (this is when the car records were put onto computer).

Also, this Wiki page is helpful at identifying the year and county of the old pre-1987 format plates:
http://en.wikipedia.org/wiki/Vehicle_registration_plates_of_Ireland