Irish ipad 2 data plans

I would like to purchase an ipad 2 with 3g in ireland. I spend half of the year in spain so i would like to purchase a spanish micro sim with a spanish carrier to use when im there and when im in ireland i can use wi-fi. so i am just wondering if anyone know's if when im buying an ipad 2 in ireland whether or not i have to get an irish data plan? if i have to get a data plan is the ipad locked to that network and would i need to get it unlocked to be able to use it with a spanish sim?.