Although this was the first and main fireplace in the original castle it has had modifications done to it, quite possibly in 1912 as the stone pillars on each side is the same material as used in the Great Hall.
Above the thick mantle there is a piece of wood that looks totally out of place. This replaced a sheet of glass that covered this area for many years as the original piece of stonework that was there fell out and was damaged beyong repair.
The stonework that was above the fireplace was a carving of the family crest just like other castles had in the same time period when they were built. It is unknown if the stonework also had text inscription but it certainly would have had the heraldic crest..
Under the fire lintel is a glass panel, this was added in the early 1970s prior to the castle opening to the public for tours. The only logical explination for the glass was for the public to look up the chimney rather than climb into the actual fire, but will leave you to fathom this mystery out.