A licence is required to search for archaeological material, including objects, if the search involves disturbing the ground. That includes all archaeological excavations which must be directed by a qualified archaeologist, licensed by the Department for Communities..
A licence application must be submitted for every excavation, by the archaeologist who will direct the work, at least four weeks before the date on which work is due to begin.
More information at: A Guide to Metal Detecting, Archaeology and the Law
Application for a Licence to Excavate for Archaeological Purposes