Imagine you are an archaeologist looking for a 250 year old four story castle. It may have been destroyed, but you're still hoping for buried walls, or foundations, or perhaps just the difference in earth which shows where stones used to be in the ground. This is where you use historical documents, maps, records, and tell tale signs from above ground to identify likely places to dig. You may also then use none invasive methods of investigation to scan the ground before you start digging.
Now imagine that after digging, you fail to find anything much by way of solid evidence. Ah well.
Now image that the four story castle is actually still standing, behind a street called Castle Street, but experts had previously dismissed it "with a cursory glance" as being "of no great historical importance". Oops...
The full fantastic story about Clones Castle being 'rediscovered' from under a pile of rubbish and undergrowth is here.