Is the extract below of any use Kenmare? From Revenue's FAQs.
You will have to look into who is the assessable person for the flat; if you are the owner of both, I'd say you can include the value of the flat in with your house.
You would make the return and payment for the house, then write to Revenue explaining what you have done and saying the flat is not separately liable.
10. I am living in a granny flat. My flat is attached to my son’s house but it has its own front door. Should it be assessed separately or with my son’s house?
The strict legal position is that any self-contained dwelling, such as a granny flat, is treated as a separate residential property that will incur a separate LPT liability. However, Revenue recognises that certain types of dwelling that are an integral part of a larger building may be difficult to value and sell on the open market. Therefore Revenue will give a liable person the option of valuing a granny flat as part of the overall building where the liable person in relation to both parts of the building is the same. However, where there is a different liable person in relation to the granny flat and the rest of the building, the granny flat should be valued separately for LPT purposes. This treatment also applies to other similar types of dwelling that are an integral part of the overall building such as converted garages and side extensions.