Be tough to do with metal detector alone. Anything small and Roman could have been brought to Ireland by traders or Celts who obtained them on England's soil. You'll need proof of encampment or structures.
is what you need to read. Apart from the possible Roman fort the most important point is that Roman finds (of which there have been quite a few) are not found on the sites where native finds are made. If Roman items were imported they would have most likely been lost at/round the native settlements.
One of the reasons for the invasion of Britain was to gain access to the gold,slaves and fighting dogs of Ireland.