Normal mortgage payments are split into an amount that goes against capital and an amount that goes in interest. For example, if the mortgage is new and your repayments are 1800 a month, the interest portion could be 1200 and the capital repayment portion 600. Your mortgage provider will give you a statement at the end of the year showing what portion of your repayments is interest and what is capital. If your mortgage is with NIB (not sure if the other banks do this), you can also access this online on a month-to-month basis.
All rental income is judged to be "income" as normal and you are taxed on this. You can, however, deduct expenses against rental income before you apply your normal tax rate. One of those allowable expenses is 75% of the interest portion of your mortgage. So, taking the example above, if your rental is 1200pm and the interest portion of your (1800pm) mortgage repayment is 1000pm, you can deduct 750 of that (75%) from your rental income and pay tax on the difference (1200-750=450pm). So, you pay tax on 450pm.
There are other expenses that can be deducted from rental income, such as your PTRB fee, insurance, repairs etc.
You need to stop your Tax Relief at Source on the property (by informing Revenue) and you need to register with the PTRB before you rent the property. You need a BER cert prior to first rental. Keep all your receipts.
I think you need to have a chat with an accountant to get information on this. As you say, you're not well up on what needs to be done and there's only so much information that an internet message board can give you.