You are liable to tax on the profit.
75% of the mortgage interest you pay and 100% of other costs can be deducted from the rent in calculating the profit.
That is 75% of the interest element of your mortgage payments. Not 75% of the total mortgage payments.
The profit is taxable at your marginal rate. You will get a personal allowance, so the actual amount of tax you pay may be low, if you have no other income.
The tenant should deduct part of the rent called withholding tax. They pay this to Revenue and you can claim it back. I am not sure in practice if this ever happens.