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.