Debt collectors in Ireland dont have the same powers that debt collectors and baillifs in other countries have e.g. UK. They cannot turn up and seize property or refuse to leave until they are paid etc. Property and money in Ireland can only be seized on foot of a court order and by the County Sherriff. Essentially these debt collectors have no power other than to remind you to pay the bill or collect payments made voluntarily on behave of the company employing them.
I usually take the approach that mistakes can happen and if people are civil and reasonable about it, then I'm happy to clear up any misunderstandings. However, if people, whos fault the mistake is in the first place, start acting up and causing me problems, if after me pointing out their mistake to them a number of times, still act up, then I prefer what the sledgehammer and nut approach as a way of teaching them to get their act together.
Firstly, as they cannot enter your premises without your permission, if they call again, I would simply tell them that they are barred from your property and that you have no relationship with their company and you have not received anything in writing from Smart saying that their company is acting on their behalf (I mean directly in post from Smart, not some letter possessed by the debt collector themselves). Tell them that if Smart want to talk to you, they should call you directly, not via a third party. If they call a second time, then I'm afraid that the Gardai will have to be called and both tresspass and harrasment charges are filed against the INDIVIDUALS who turn up (not the debt collection company - always better to go after individual employees as gets better results due to individual not wanting permanent record).
I would then get your solicitor to write a letter to Smart outlining what has happened and what was previously agreed (i.e. that you wouldnt be charged). Also include in letter than you have been harrassed without reason by third parties and that you have an issue with your personal details been passed onto third parties, particularly since this information is incorrect and portrays you in an unfavourable light (their legal people will know where this is going). Ask them for a written apology outlining that they were incorrect to state that you owed money and that they regret passing on information to third parties etc.