Well, if you invest in a US domiciled ETF you will have to pay income tax at your marginal rate (plus USC and PRSI) on all distributions received and CGT on any gain whenever you sell. US funds are required to distribute all dividends and realised capital gains.
Alternatively, with an Irish/EU domiciled (accumulating) ETF you simply pay 41% exit tax on disposal, with a deemed disposal after 8 years. It's worth noting that if you sell after 8 years for less than the value of the ETF on the deemed disposal date, Revenue will refund you the difference.
Which is the better option for you depends on your marginal effective tax rate and whether you are carrying losses forward (or might generate such losses elsewhere).
As a matter of interest, is there a particular reason why you want to invest in a fund that tracks the S&P500?