my personal point of view is that you take a short term mortgage, not an interest only... If you can buy a place with a 15-20 year mortgage every cent of rent afterwards is yours (ok less what revenue will want).
Why should someone pay you €9k per annum for the same apt that is rented out for €8k elsewhere... rent really isn't dependent on what mortgage you have set up. After all the bank doesn't expect your tenant to pay your mortgage, they expect you to... so if you overprice your place and can't get a tenant you are the payee...
As for the dropping rents at the moment, I heard a statistic a year or so back, that something like 60% of the renters in dublin worked in construction (ie a lot of the english, polish contractors), so if we are now in an economical downturn, a lot of these worker are very mobile and can just move out to go to the next construction hot spot... so many places seem to have an oversupply of rental properties.
If you have done your research, and the sums make sense you would can come out ahead in the buy to let business, but research, researchy, research...