We only have 1.5 days in Dublin and given the state of air travel don't have much degree of confidence we will arrive when scheduled (10:30am). We're cramming a lot in to those 1.5 days, and on our 1st day (the 1/2 day) we hope to see Dublin Castle and the GPO plus other stuff that's just part of the North of River Rick Steves walk (nothing else requiring tickets) before doing the Musical Pub crawl at 5:30. We have Kilmainham Gaol already booked for the next day at 9:45, then plan to see 14 Henrietta Street after that. So my question is - is it really necessary to pre-book to see Dublin Castle, the GPO and 14 Henrietta Street? We'll be there Sept. 5 & 6.
I just took a look at the booking sites for Dublin Castle, the GPO and 14 Henrietta. Both Dublin Castle and the GPO have plenty of slots open so I don't think you will need to pre-book. Just show up. You might have to wait a few minutes for a tour to start but that's all. 14 Henrietta is a different story. Without knowing when you'll be in Dublin (but I assume soon since you already have Kilmainham Gaol tickets), you may not be able to just show up. I took the tour in 2019 and thought it was excellent. It really made me appreciate Dublin and Irish history and life. It's a very small group tour and from the booking site it's clear that certain days and times sell out. I will also point out that it is on the north side of the River Liffey, about a 15 minute walk from the GPO. Having been to both Dublin Castle and 14 Henrietta I would prioritize 14 Henrietta over Dublin Castle.
We were not very impressed with the Dublin Castle, does not even look like a castle.