Schengen just means that there is no border control at the border of other Schengen countries. People from other EU countries can still enter and live in non Schengen countries, they just have to show a passport. That is not really control of your own border.

If 5 million people decide to move from Finland to Ireland in the next month there's nothing anyone in either country can do to stop them. One is schengen country one is not. Both are in the EU.