sse,
The text I wrote is quoted directly from the PLanning & Development REgulations 2001-2007. As you can see it does not refer to 'the reconversion of converted flats' as you state, but states simply the 'change of use from use as 2 or more dwellings, to use as a single dwelling' The word used is 'dwelling'. Under the Planning & Development ACt a house is defined as a dwelling along with flats and apartments. So why you think it only refers to changing flats back into a single house I can't understand.
Certainly if it is a protected structure or there are external construction works beyond that which are exempt are required then planning permission will be necessary. And by all means an expert such as a structural engineer would be needed to look at the structural issues involved of knocking the walls.
Also sse, what exactly do you mean by building density considerations? Surely the same floorspace as has always existed on site will remain unless he is extending the property?