Hi,

Welcome to OSM. :)

OSM is a volunteer community effort. We have limited access to aerial photos and out-of-copyright maps. Everything else comes from what the volunteers can learn themselves from surveys and map.

building=house means the building was **built** as a house. If we knew that it is current used as something else, we can add building:use=commercial.

We don't have access to the EirCode database, RTB database or internal drawings or other documents of any kind. We simply have no way of knowing that it had been sub-divided. 

Even looking at estate agent photos from last year, it would be difficult to definitively conclude it had been subdivided at some point. The one clue I did see was the wash hand basin in one of the bedrooms.