How to amend entry stamp?

If someone arrives in Dublin from outside the EU they will get an entry stamp. That stamp will say that they can stay for upto 3 months but the immigration officer also asks how long they intend to stay and if they say a week then the officer will write the date of intended departure in pen on the stamp. Does anyone know how you get that extended? Have someone here who needs to stay an extra week - so longer than the handwritten date but we’ll within the 3 months allowed.