A PN: Greenland leaving in the early 80ies was - despite Greenland not being a member state - often referenced as an example of how a member state could leave the EU. So even before Lisbon, leaving EU was a possibility. The GFA didn't take it into account, but not because it was impossible. Presumably the negotiatiors had more pressing matters to attend to then what to do if - in a distant future - the UK or the Irish republic would leave the EU.