The law says it is illegal "to use a detection device to search for archaeological objects anywhere within the State or its territorial seas; without the prior written consent of the Minister for Culture, Heritage and the Gaeltacht"
What if you are searching for gold, lost items, underground cables, or other, and you stumble upon an archaeological object? You were not hunting those objects, so I would say you were within the law. What you do with the fortuitous find is another matter (IANAL).
My brother and I were the first to buy a house in our immediate family. My Uncle Vic was the first to say you never really own your house or land. You pay property taxes after you have paid off your house. Your land can be taken with little compensation in some matters, etc. I can see why you would be happy and worried at the same time discovering artifacts whether you were looking for them or not.