That is partly correct, the windscreen modifications applied from ALX400 builds onwards are to reduce reflections from internal lights.
I have never driven any of the Dublin Bus deckers so can't say how effective the mods are but it is certainly an issue that is a problem on dark roads in standard buses. While Dublin Bus order these modifications sister company Bus Eireann (who I work for) do not.
It is not the case that the standard Gemini 3 has been changed to suit DB orders, they still produce a custom batch. The windscreen arrangement is still very different as in these pics.
https://www.flickr.com/photos/be216cd1/17190704136/
https://www.flickr.com/photos/be216cd1/16141227104/
Wrights have been quite flexible with Dublin Bus, as well as those mods they continued to produce pre-facelift Gemini 2 bodies as DB requested the old style panels. Much of this is down to their eagerness to get the orders from a company that had only bought Alexander/Transbus/ADL deckers for nearly 20 years.
As a publicly owned company the orders go out to tender so manufacturers have to meet the spec to qualify, once they met that it was probably more the poor quality of the initial E400s in comparison to the Geminis that swung the later orders to Wrights.
Sadly the Mallusk plant closed 11 years ago with the demise of Transbus, it was never part of ADL.
That is from the initial batch of ALX400s for Dublin Bus which did not have modified windscreens, AFAIR they did have curtains on the cab door which could be closed over the attack screen to block out the light. Later batches did have the modified windscreens.
https://www.flickr.com/photos/be216cd1/16731971856/
BTW, they are not my photos, check out that guy's flickr page for lots of Irish transport pics, road and rail.