This happened us a couple of years ago when we were selling. A small chunk of land where trees were planted at the far end of the back garden were still under the original land sellers folio. Funny because it got missed twice, first when the land/house was originally sold, then again when the homeowner who sold us the house. Thankfully for us, the original seller lived two doors down from us, so we chatted through it, and he seen it right away and had no issues with it. He did go to his solicitor to get it looked at properly, and when agreed, our solicitor chatted with their solicitor and sorted it out - yes, the original seller (who technically owned the piece) had to sign something putting it in our name. From memory, I think it cost €150, basically the other solicitors fee for getting the folio checked & new paperwork.