Another point to keep in mind, outside Ulster, there was very little industrialization of Ireland in the 19th & early 20th C. If you make Irish a national culture for ENG, the AI will of course turn Ireland into an industrial powerhouse, and with V:R's capitalist system, there is no way to mod the game so that it does not industrialize areas that have national cultures.
So in the end the question is as much about game balance as anything else. ENG is already very powerful in VIP:R, would allowing it to turn the 8 million Irish at the start of the game into industrial workers really be a wise thing, given that the modding team does not have any way to control what the AI does with them once you make Irish a national culture? Especially since it is pretty much impossible to model the demographic collapse caused by the famine, making Irish national culture for ENG is really giving it a huge new power source that would make the development of Ireland in game even more ahistorical than it already is.
So in the end the question is as much about game balance as anything else. ENG is already very powerful in VIP:R, would allowing it to turn the 8 million Irish at the start of the game into industrial workers really be a wise thing, given that the modding team does not have any way to control what the AI does with them once you make Irish a national culture? Especially since it is pretty much impossible to model the demographic collapse caused by the famine, making Irish national culture for ENG is really giving it a huge new power source that would make the development of Ireland in game even more ahistorical than it already is.