Depends on what the money is used for. For example, if it is used to purchase or renovate your PPR then the interest should qualify for tax relief. Not sure if the CUs facilitate granting of this relief at source though. If it is used to purchase or renovate an investment property then the interest can be set against rental income as an allowable expense.
I'm not sure about TRS, but my credit union gives me an interest rebate on interest paid on loans each year, I think it's approximately 27% of interest paid...
The CU interest rebate is nothing to do with tax relief. It's just a discount on the normal interest rates that some CUs give at the end of the year. Any loan used to purchase or renovate a PPR qualifies for tax relief on the interest up to the usual TRS limits. I just don't know if the CU can do this at source.
Any interest paid that is not covered by TRS can be recovered directly from Revenue subject to the max allowed. Just contact Revenue and they will increase your tax credits accordingly or wait until the end of the year and make sure it is included on your balancing statement.