Have you read the Property Investment FAQ in the Property Investment forum which summarises some of the issues involved? If you don't know then you really should get independent, professional advice on the tax and maybe general investment issues involved.
First thing is tenancies must be registered with the PRTB (Private Residential Tenancies Board @ www.prtb.ie ) This is a requirement of the Residential Tenancies Act 2004. On Tax, you should at least check Revenue leaflets and guides available from www.revenue.ie ) regarding what to do, what expenses are allowable etc. Rental Income is "Self-Assessed" meaning you have to tell Revenue about it, you can't wait for them to come to you. Tax on the Net Rental Income after expenses will be chargeable to Income Tax/PRSI/Levies at your marginal rates, i.e. if you already have exposure to tax at 41% then you can expect that rate to apply to the rent also. However as mentioned there are a lot of issues and a visit to a tax professional would be worth the money