If the amount of the plaster splashed into your eyes is only a little, you can just solve it on your own, such as washing your eyes with much water to make those plaster out of your eyes. But if there are a lot plaster, and you feel pain, you'd better see an emergency physician before it become worse, because Plaster is a highly alkaline substance, which would cause your eyes at significant risk.
Well, it is really unlucky for you to have got that problem, since that would be really painful. However, you should not stand around and do nothing. For the time being, you must try to take it out of your eyes without rubbing your eyes because this is dangerous. But, if you don't manage to take it out, then it would be necessary for you to get some professional treatment in a hospital or eye clinic.