Off the cuff this sounds like another classic episode of the long running series of "you are not getting your deposit back".
I share the doubt about RTB jurisdiction especially as there was no formal lease.
Ergo, if there was no lease one wonders what, if anything, was registered with the RTB.
The landlord should have registered the tenancy as failure to do so leaves him open to prosecution.
I would ask the landlord out straight if he registered the lease with the RTB reminding him, before he answers, that he faces prosecution if he has not done so and BTW could I please have my €600 (?) back ? I would do that one verbally and not in a letter to avoid leaving yourself open to an allegation of making "unwarranted demands with menace" which is actually an offence. ( An unwarranted demand with menace is actually the old concept of blackmail !.)
If the tenancy was by any chance registered the third party issue raised by Leo arises.
Who was the "legal" tenant ?
If the occupant employee was the de facto tenant that may cause a problem as it means OP was not the tenant and the issue of privity of contract may therefore arise and cause the RTB not to deal with the OP. The only way around that would be for the OP to effectively use "subrogation rights" whereby they pursue the issue in the name of the "tenant" but that would require prior agreement with the tenant.
GENERAL OBSERVATION and LESSONS.
OP obviously went about things nicely in arranging the accommodation but then gets this slap.
1. Always do a "photographic survey" of a rented property before taking up occupancy as that sorts out end of tenancy disputes.
2. Always get a formal lease to protect yourself as a tenant.