Well, taking your first point re 'strains' between growth here and growth in mainland Europe. This is no big issue, it happens all the time in large currency areas - e.g. in the US New York, California boom, while the rust belt states suffer. This doesn't lead to anyone claiming NY or Cali should leave the Union. Same thing in the UK - South East booms, scotland, wales etc. bust has been the pattern of the last 10 years.
(Although there is an argument that in particular the US labour market is far more flexible i.e. if no jobs in Arkansas, people will move their families to Cali or New Jersey etc. far more easily and readily than Germans will move their families to Dublin).
On tax harmonisation, the EU (read France and Germany) have been demanding tax harmonisation for some time. However, Ireland and Britain have consistently blocked attempts to even get this to the table. Now that the 10 accession countries are in, the 'states rights' countries have only become more powerful, not less. Therefore tax harmonisation is even further down the road than before. Tax competition is rife, and works extremely well, amongst states in the US.
Secondly, our economy is not solely dependent on building Dell boxes in Bray - our financial services industry is easily as big an employer (and arguably a better employer ie higher wages, better prospects) as manufacturing for US multinationals. Ironically, whereas the much promised high-end R&D jobs have for the most part failed to materialise in software, pharma etc., the Irish financial services sector is starting to aggressively move into 'front office' financial products, and is seriously threatening Luxembourg for instance in value-added funds etc. Also, don't forget agriculture is still a huge industry, dwarfing US multinationals in the numbers it employs, etc. etc.
Also, where would the US multinationals go? If the whole of Europe moves in tandem, where's the advantage in going somewhere else in Europe? History has shown that labour costs in 'cheap labour' economies rise fast once the economy's wealth increases. What if they move to Turkey/North Africa? Hardly likely - language barriers, lack of sufficient numbers of skilled operatives etc.
If Dell, MSFT, etc. want to benefit from EU trade agreements etc. they'll continue to locate here, where we have the most flexible labour laws in the eurozone, as well as the only native English-speaking workforce.