Can I change a ready-made itinerary?
That is what they are for. Open any itinerary, from the 3 day Dublin trip to the 14 day full island route, and use the Open in the journey planner button: its stops load into the planner as your starting route. From there you can swap stops, slow a day down, take a stop out, and add places of your own from the 189 towns and sights in the planner, including the place your grandmother came from. The days, overnight bases and drive times rebuild around every change. The ready-made version stays untouched on its own page for the next person.
Useful pages: Open the journey planner · Ready-made itineraries