Upvote:1
The page Traveller cites answers the question, but is confusing.
Restated: you won't need an Irish visa if all the following is true: a) You arrive and depart DUB at Terminal 2, b) Your arriving flight arrives and your departing flight leaves between 0400 hours and 1600 hours, and c) You arrive and depart on the same calendar day. Each transit (westbound and eastbound) is analyzed separately.
Here's a screenshot of the page in question: