The full amount of the interest can be offset againt rental income. i.e. in your example, your rental income is €9000. Your deductibles are €8400 interest, and whatever the depreciation and insurance costs are. In fact, it sopunds as though you'll be in no better than a break-even situation, and maybe a loss.
You *must* be registered with the PRTB (and have to confirm this and produce evidence in case of a revenue audit) to be allowed offset interest against the rent received, but the cost of registration is also deductible.
Revenue told me [edit: inaccurately] that for rent in excess of about €3000, a Form 11E should be returned instead of a Form 12.
Edit: Yes I was surprised, yes I did query it - but had since decided that I was submitting a Form 12 anyway since it's just more appropriate. Cheers, webtax and DonKing for validating that for me!