Plenty of risks. The first things to check: is it stolen? is it locked to a particular network?
If it is stolen and reported stolen, the phone, through its IMEI, will be placed on a blacklist which will not allow it to be used as a phone in ireland.
If it is locked to a particular network, it will only work with the respective network unless you can get it unlocked or jailbroken.
I would stay away from Donedeal and the like.