You may need advice from a competent structural engineer or an architect experienced in this kind of work.
Of course, you may simply be somone who won't tale good advice no matter what the source.
If you cannt be dissuaded, you need to seriously think this through.
Considerations of implications should include; -
- Will the visible portion serving your flues remain?
- If not, you may need planning permission for the change.
- If yes, you need to work out how to support this with its lower sections gone.
You cannot leave unresolved loads affecting your neighbours chimney and gable wall - if it fails, it may tend to fall on you, not them!
More important, if these rooms have no permanent vents and are not "blessed" with one of the New House Mech Vent Heat Exchange thingies, you may need to vent the rooms.
If you'r einstalling gas fires, you need to talk to Bórd Gáis about the vent requirements, but it is an additional venting requirement, "as well as" not "instead of".
Personally I think you're going to a lot of trouble for next to nothing.
Chimney Breasts are an expected part of Period Houses and add to their charm.
More importantly as a notable characteristic in main rooms, their absence may lowere the house value.
If this is a listed building you'll also need permission to remove any part of the fabic, particularly a feature like this, or the upper external section.
If this is a modern house they won't have much of a chimney breast projection to remove and you're going ot a lot of work for nothing.
Just run shelves between the breast and the walls and select a mirror or a picture for the face of it.
Put some feature lighting in the alcoves and place some spots highlighting the picture.
There is nothing worse than a featureless main room.
You may as well go and rent out the house and live in a shed in the garden.
ONQ