You'll have to give them notice of 224 days, so they won't have to leave until 25 October if you give them notice today. You can only give notice if you need the property for yourself or your family member or if you intend to do major building work or sell it. Otherwise they can stay on for another (I think) 2 years until the end of their second Part IV cycle (4 years plus 6 years).
Check RTB website and make sure you follow the process exactly. If they decide to move out sooner you can agree that between you. In fact if they decide to move out today without even telling you there's very little you can do about it but if you don't follow the procedure properly they could get a massive compensation payment from you.
It is surprising that someone has been running a business for 8 years with apparently little knowledge of the laws that surround it.