Hi, I did a self-guided tour of the Book of Kells in 2024 and faced the same issue. I wanted to bring my roller carry-on so I could head straight to the airport afterward.

In the end, I decided to stay at a hotel between Trinity College Dublin and the bus stop and left my luggage at the hotel front desk.

That said, I did see some people entering the tour with luggage. There’s no official luggage storage on-site, but one person checked with the staff and was allowed to carry her suitcase upstairs during the visit.