Firstly, have you issued the 14 days rent arrears in compliance with the law, i.e have all the details that are required been done correctly? Secondly, the Notice of Termination must also be completed as per the law requirements. Failure complete either correctly will make the NoT invalid and if you try to evict on the basis of an invalid notice it could well cost you in the thousands of euros even if you believe that it was correct.
Landlords are suppose to know how to complete both these Notices correctly, unfortunately, many do not and in a claim brought by a tenant the tenant is often awarded hefty damages.
It is often useless to pursue tenants who have defaulted in rent payments. Firstly, you need to know the tenants current address, secondly the tenant may not have any money (especially if they are on SW). Thirdly, many landlords think that the hassle involved is not worth the end result - the landlord wins the claim but the tenant, for what ever reason, does not pay or the landlord is awarded a few pounds per week which would require such a long time for the debt to be repaid that it is not worth the effort.
By all means contact the PRTB and see what they have to say but I doubt they will help greatly.
As soon as rent is overdue, landlords should immediately issue a 14 days notice of rent arrears. Never let it drag on or the arrears will mount up as you have discovered to your detriment. However, it is vital do everything exactly as the law requires. Keep copies of all correspondence and either have a witness if you deliver any notice to the property by hand or have dated photo or video, or sent all correspondence by registered post. IMHO, registered post can always be refused or the tenant may "act" as though he is out when the postman calls.