I'm fairly certain that "anywhere" bit is wrong. You only need a license when doing it on public land, except for scheduled monuments, historic monuments, and a

I'm fairly certain that "anywhere" bit is wrong. You only need a license when doing it on public land, except for scheduled monuments, historic monuments, and areas of archaeological interest which require special permission as well. If it's on private land and you have permission from the landowner, you don't need the license. You still need to report any treasure or archaeological finds to National Museums NI though.