James,
Planning requirements are usally to submit a 1:2500 'place' (planning, legal, agricultural, construction and environment) map. These can be obtained from Ordinance Survey Ireland for around 40 eu. It will show the area of the plot in Hectares which you can convert to Acres.
Also, and very important, go to see land registry, they will have more detailed information on boundaries etc. Boundaries of site can vary from hedge lines, walls, road edge etc. Land registry will charge you around 10 eu to veiw any documents you like and they can be very helpfull.
Finally, if you are still unsure as to the area of land, you can employ a land surveyor who will do a measured survey of the physical boundries and any features you may need for planning, ground levels etc.