I recently purchased a house in Costa Rica. I have an interest only loan for which I would like to claim the interest on my US tax return. I am a US citizen and this is a second home.This is a private loan for which there is a mortgage contract. Will I be able to claim the interest paid? Will I be subject to the 30% tax by the IRS.

YOU are not subject to the 30% tax; the person to whom you are paying the loan is subject to that tax.

You ARE legally allowed to deduct the mortgage interest, but since you are paying it to a foreign national, the IRS MAY require you withhold that 30% if you claim the mortgage interest on your tax return. If you have not withheld it, they will require that you pay it out og your pocket and recover it from the foreign national, who would probably object to that.