The answer to your question is that you can indeed to your Instrument Rating and/or Multi-Engine rating in a country other than the UK, as they are ratings and therefore seperate to the Commercial Pilot Licence. The issue with the ATPL's is that they are an integral part of the commercial licence.
Incidentally, your initial assumption that after doing your ATPL exams with Bristol you therefore need to do your CPL in the UK is no longer correct with regards to doing the flight training in Ireland, as of about two weeks ago.
See this thread....
UK / Ireland Reciprocal Agreement