If I was in the OP's shoes, I would change the locks and try to re-let the property.
The property has been vacated, with rent outstanding, so this is perfectly legitimate.
Section 37(2) of the Residential Tenancies Act 2004 provides that a tenancy will be deemed to have been terminated by the tenant upon any rent owed by him or her being in arrears for a period of 28 days or more if -
(a) whether before or after the end of that period, the tenant has vacated the dwelling; and
(b) no notice of termination has been served by the tenant in respect of the tenancy.
That provision has not been impacted by The Emergency Measures in the Public Interest (COVID-19) Act 2020.
The OP can obviously retain part or all of the deposit to cover the rent arrears. I would write off any arrears over and above that amount.
It probably makes sense to advise the tenant of this course of action as a matter of courtesy.