It is not established that the tenant has done anything deliberately and with intent, honestly it looks like a daft thing to do as it would be spotted.
If the tenant is involved I am saying use this as leverage to get him to move out, the OP will know more after a face to face conversation, my priority would be to get the property back in good shape as trust will have broken down.
As for rushing to report it to the Garda, not the priority right now for the owner of the property, anyway, there will be no blues and twos in terms of an urgent investigation post reporting, this is not the biggest crime the Garda have to contend with, getting the Garda involved will entrench your tenant and you will ultimately pay for that.
Do your own investigation and get the tenant to move using the information you will gain from your discussion to your advantage, the tenant can break their lease without a great deal of palava involving the RTB