The electronic rat zapper is specifically designed for killing rats. It sometimes works with ground squirrels, that is, squirrels that live in burrows, but it is not likely to be very useful for controlling the species of squirrels that live in Ireland. However, Havahart products can be very useful for controlling squirrels, provided you are willing to dispatch the squirrel to its eternal reward when you check the trap.
In the Republic of Ireland, the native red squirrel is protected, and since it feeds on nuts and seeds, it is not much of a nuisance. The imported American gray squirrel is not, but in most counties it is a criminal offense to catch a gray squirrel and release it back to the wild. You will be expected to kill the squirrel with a shovel or a hammer, and bury it yourself.
Why not just use a spring trap? The problem with using spring traps in Ireland is that they kill protected red squirrels as well as nuisance gray squirrels. It is lawful for you to use a rat zapper to try to kill gray squirrels, red squirrels almost never venture near one, but you will get much better results with a kill-it-yourself method.