I don't know anything for certain but, if I were in your shoes, I would travel to the southern part first and try and unearth any details of your father's title. This might involve employing a solicitor if your efforts do not bring out help from the officials. Remember that (unless you know otherwise) it may already have been disposed of by your father. Then once I had the details of the location, etc. I would travel to the north and try to locate the plot - be prepared to find that it has already been built on. Finally, the challenge is to persuade the northern authorities to accept your claim to the land. If you find little or no sympathy then probably the only course you might consider is to turn to the EU and see if they are willing to listen to you and offer advice.