You will need to make an inventory of the furnishings in the house and, if they haven't been specifically purchased for the letting and you don't have receipts, you will have to estimate the value of the items (current market value, not replacement value).
Once the estimates used are fair and reasonable, there should be no problem with Revenue.
You can deduct 12.5% of the value of the inventory each year over 8 years.
If you have to replace a suite of furniture for example after 3 years, you can write off the remaining value of that suite and add the value of the new suite to the inventory.
You can also write off kitchen, bathroom and household accessories in total in the first year and any subsequent year in which you have to replace them. For example, if you purchase a set of saucepans for the tenants, these can be deducted in total from the first year's rental income as they are not considered re-saleable items.
Hope this helps!