Does a Papua New Guinea passport holder require a transit visa for a 2-hour stopover in Dublin on the way through to Edinburgh?
From the Republic of Ireland site Department of Foreign Affairs and Trade listing visas for Ireland, under
Requirements the link to the Irish Naturalisation and Immigration Service Check if you need a visa will answer your question.
Papua New Guinea for
What nationality is shown on your passport or travel document?, and then selecting
Transit through Ireland. eg via an Irish airport for
How long do you want to stay in Ireland?, gives the result:
You do not need a transit visa
You do not need a transit visa to travel through an Irish airport on your way to another destination.
Note that if you actually wanted to be a tourist in Ireland you would need a visa.