It's great to hear you are visiting Ireland soon!
Your trip sounds amazing, I love to see you have your whole itinerary planned. To visit all these places without a car you will definitely need to use both trains and buses (and sometimes taxis depending on the time of the day), as some places, such as Hill of Tara, Newgrange and Carlingford (among others) and not very accessible by train. The Leap Card is definitely your best choice when traveling around Ireland, as you will be able to save some money and there are no passes available for Bus Éireann (such as the Open Road pass you mention). You can plan your trips using the Transport for Ireland
website, the Bus Éireann
site and the Irish Rail
Regarding places to visit on the island, have you checked our trip ideas website? https://bit.ly/3b5fBN3
Perhaps you could find some more places to visit from here. For example, I don't see you are going to Cobh
and you might like it, it is the village where the Titanic stopped for the last time before leaving Ireland!
I hope this is useful and hope you have a great trip,