The first thing I noticed when checking the gallery of photos in the Coventry Telegraph is that the rest of the photos clearly show that whatever the effect is, that it is also reflected in the floor of the hall.
The Green "apparition" reflected in the polished floor.
A little research revealed that the tapestry behind the Irish President is protected by a special form of glass which protects the piece from certain wavelengths of light. I think the "phantom" is simply light from one of the attending TV crews studio lights. In the top photo, the outline of a chair in front of this glass clearly makes up the right-hand side of the "apparition".
Its due to this that I think this apparition is simply light from studio lights, reflecting off the protective glass and then on to the polished floor. Could be vice versa. It's something for us to REFLECT on! Sorry...