Roman coins were found at Newgrange and it is assumed from this that some Romans came site-seeing. The other alternative is that some traders obtained some Roman coins and then offered them to Newgrange or the spirits thereof. I think it was George Bernard Shaw who planted Roman coins at Tara when the British Israelites were digging for the Ark of the Covenant.

A few swords etc have been found, but these again were probably traded. None of the Roman historians ever chronicled a major jolly to Ireland as far as I know. It's a long trip from France directly to Ireland (17 hours by modern ferry), so the only practical route would have been via the west coast of England, Wales or Scotland. Being as the Romans had there hands full in each of these areas the oportunity of making a major crossing wasn't really open to them.