This is hypothetical.
If a credit card company, for example, sold your debt to an agency, and it was up to the agency to get back their money, what can they do, within the law, to try and retrieve the money from you?
What is their typical approach?
Buying of debts is very rare in Ireland. Legally they must give you a notice that the debt has been sold and to whom. Normally in Ireland the debt agency just acts on behalf of the bank. The debt is not sold.
If they do own the debt they can sue you in the courts for the money after sending a few letters first. Once they sue you they can send the sheriff to seize your stuff.