I can't answer the part about receiving training in Ireland to add a night rating to your UK CAA JAR PPL. I simply don't know the CAA's position on that.
The part about flying in the UK at night on a CAA licence will depend on if your aircraft is an Irish registered aircraft or a UK one. If it's UK, then there is no problem....the only difference between you and everyone eles in the UK, is that you live in Ireland. But your aircraft, licence and position would be the same as anyone else in UK airspace.
If your aircraft is EI registered, then the answer is no. The reason for this is this.
IAA Rules of the Air Order 2004. The relevant section is 38( 8 )(iii)
Flights by night outside a control zone shall be operated as IFR flights in accordance with Part IV of the Rules in this Order unless otherwise prescribed or authorised by the Authority.
So you must be able to fly under IFR. No problem for most of us, as our JAA PPL doesn't prohibit us from flying under IFR....it simply prohibits us from flying in weather conditions which require us to fly IFR. So it's perfectly legal for us to fly under IFR so long as we stay in VFR type weather, though there is generally no point in doing so. The UK has a similar rule, so all night flight in the UK (outside control zones) is actually under IFR. Those without IMC's or instrument ratings in the UK can quite happily fly IFR at night, so long as they stay in VFR type weather.
So far so good....you just happily fly at night under IFR and stay in good weather. And that's fine if you're in a UK registered aircraft. However, here's the catch for those in an EI registered aircraft.
IAA Personal Licensing Order 2000. The relevant section is S19 (1)
A person shall not act as pilot-in-command of an aircraft or as co-pilot of a multi-pilot aeroplane or helicopter registered in the State under instrument flight rules unless such person holds a valid instrument rating -
(a) issued or validated by the Authority and endorsed in or deemed by the
Authority to be included in the licence held by such person or in the
validation of such licence, as the case may be, and
(b) appropriate to the category, class or type of aircraft flown.
So if you want to fly an EI registered aircraft under IFR, you must have an IR rating no matter what, no matter which country you are in.
And before anyone gets the clever idea of flying at night in the Republic in a G registered aircraft, there is a similar paragraph banning IFR flight without an IR in an aircraft registered in another country, while within the state.
So while not legal, I'm not too sure just how concerned the IAA would be about someone flying an EI reg aircraft at night in the UK. Perhaps someone putting a good case to them would be able to get an exemption?