An assignment when breaking a legal binding lease contract is allowed provided the landlord does not incur any expenses.
You have the landlord's permission, you find the replacement tenant at your expense who may be referenced (vetted) and accepted or denied.
Once an acceptable replacement has been found, the Irish way is rather lax and deposit passes from incoming tenant to out going tenant.
To perform an assignment in full legality, a deed of assignment should be completed and signed by ALL relevant parties, the landlord, the out-going tenant, the in-coming tenant and any other tenants who may be in the property and would be on the same lease (if any).
Finally, the landlord should return your deposit and a new deposit collected from the replacement tenant. After all, only you are entitled to your deposit and not anyone else (though a deed of assignment could contain a clause whereby the ownership of the deposit passes from the out-going tenant to the in-coming tenant and the incoming tenant gives you the value of the deposit.
Finally, as this is an assignment of a lease, the same RTB registered tenancy remains therefore, there is no new RTB registration and the lease continues without interruption. However, the landlord may wish to have a new tenancy with the new tenant therefore, IMHO, he is responsible for a new tenancy registration fee. One of the advantages for a landlord using an assignment is that there is no new tenancy registration fee.