I think plan now to come in the middle of June to Dublin. Spend 2 weeks between Ireland and Scotland. Spend the first weekend with older DD and see Ireland, then fly to Edinburgh with the younger one. We first will do some day trips from Edinburgh or go somewhere using bus/trains. Then older DD will fly to spend 3 day weekend with us in Edinburgh; we will get back to Ireland and fly back home leaving older one to study for another half of the semester 
Do we need to buy Ryanair tickets in advance or not necessarily? Is anybody familiar with buying tickets for low-coasters for short distances?