This is likely to have occurred following possible rising damp in the past from the fireplace hearth or moisture penetration from the chimneybreast. Was the wall re-plastered following the previous work or is it still the original plaster? Migrating salts usually lead to disintegration/breakdown/damp staining of old and sometimes replacement plaster. Obviously this needs a specialist on site inspection to diagnose whether the problem is caused by penetrating damp, water leaks, condensation, rising damp or whatever.