My understanding is the following (but really, you probably need to take professional tax advice):
1. If the mortgage was taken out exclusively to fund the purchase and refurbishment of the rental property, then interest on that mortgage - no matter what property it's actually secured on - qualifies as a deductible expense against the rental income.
2. You cannot claim the interest prior to first letting as a deductible expense - Revenue is very clear on this. However, expenses incurred prior to first letting may constitute allowable expenses for the purposes of calculating your capital gains tax liability on ultimate sale of the property. Keep your records.
I'd seriously encourage you to take professional advice. It could save you a lot of headaches in years to come.
... this should be in the property investment forum, by the way.