I`m from Ireland and whilst you would get away with it, it is illegal. Also, you`d never see the newts again. If you really want to keep something outside, there`s probably no harm in axolotls (I`ve known people to do it). No one is going to mind about the axolotls because they`re not going to pose an invasion risk.
Mind you I have often wondered why someone doesn`t get a group of German fire salamanders and let them go in the Glen of the Downs or something. I think that would be an interesting experiment. A toad, a frog and a newt do not an amphibian population make...
Actually what would do really well would be Mesotriton/Triturus alpestris and Triturus marmoratus, particularly alpestris.
Disclaimer: I am in no way condoning the breaking of any laws of the Republic of Ireland.