If you go to the US, you will only have a US cpl/ir, not usable in the UK - unless converted, ie more money, it will also be harder to convert as its harder to fly in the uk...
a) as per other comments here, the laws of physics, as they relate to aviation, apply equally both sides of the Atlantic.
b) It's a small point, but the assumption that everyone on this forum wants to fly in the UK is very irritating, especially when the lad has said he lives in Cork. Maybe his dream is to fly for EI?
c) There are plenty of schools in the US which provide JAA training, under the authority of the UK CAA.
d) even if you had an FAA IR, conversion doesn't necessarily mean "more money". As an FAA IR is significantly cheaper than a JAA IR, it
may be just as economical to attain your FAA IR and then convert it to an IAA or CAA IR. Obviously, this will depend on the quality of instruction plus the students aptitude for instrument flying.