You can rent out under the rent a room scheme, earning up to €7k per annum without being liable for tax or loosing your FTB tax relief. However, you do need to be living in the house at the same time! Otherwise, as previously stated, if you rent it out in its entirity within 5 years of purchase, there will be a claw back element to the stamp duty, payable at invester rates rather than owner occupier rates and you loose FTB tax relief! You can sell the property within 5 years however (as an owner occupier) without a claw bank of stamp duty applying!
As such the rent a room scheme would appear to be your only option of buying and making some income at the same time, without effect FTB benefits!