Ireland is in the EU and UK is out, so for example entrance requirements for EU citizens entering Ireland and the UK are different, moreover there's no immigration at︀ any airport when you leave Ireland or the UK for that matter. And when you︁ travel from Ireland to the UK there's no immigration control either. So you can fly︂ from Dublin to London and then to any 3rd country and in theory no one︃ will know when you left. Don't know all the rules about the common travel area,︄ simply stating the facts.