Weight Restrictions and Dublin Bus

I notice there are new(er) weight restriction signs at Blackhorse bridge in inchicore. They are larger and state the previous 3t weight restriction. The previous signs were a truck with 3t on the side and a red line through them (see google street view). These ones have 3t in a circular sign without a red line. I presume they were changed for greater visibility of the restriction, or maybe the sign itself has been changed like the no entry sign has been recently.

Dublin Bus have three routes going over this bridge I believe - the 13, 68 and 69.

I assume a double decker is well over the 3 tonne limit. Are they exempt, or are they committing an offence by having routes over the bridge?