Nope, you've complied, or more particularly you haven't failed to comply, with the Residential Tenancies Act.
So once the property is registered with the PTRB, regardless of who registers it, then the obligation is met to allow mortgage interest to be offset?
This all assumes it definitely isn't your responsibility to register. If it's someone else's responsibility and they mess up, their problem. You haven't broken the Act so you're in the clear. But if it's your responsibility to register, it's your problem.
The availability of the interest deduction is subject to compliance with the PRTB registration requirements. However, a dwelling let by or to a public authority (including a local authority) is specifically exempted from the registration requirement under the Residential Tenancies Act 2004.
In other words, if you are in compliance with the statutory registration requirement (because an exemption is applicable) then 75% of any relevant interest payments can very clearly be deducted in calculating your taxable rental income.