train from dublin to belfast

It's not clear what day of the week it is, as there only seems to be a 1pm from Dublin on a Sunday, and on no day of the week is there a 6.20 back from Belfast.
If it's a weekday there is a 1.20 from Dublin and a 6.05 back from Belfast. If it's a Sunday it's 5.05 and 7.05 back from Belfast.
The trains cannot sell out- they are mass transit. But unbooked passengers may have to stand.
An on the day day return fare is 32 Euro

If you book ahead each leg is 15.99 Euro (so same fare, bar two euro cents) and you can cancel or amend up to 60 minutes before departure, and you get to choose your seat on a plan. So there is no real reason not to book ahead.
If it is on a Sunday then the 1pm from Dublin is an extra train- so is not the usual Enterprise cars- just a normal Irish Rail inter city train (still very nice trains).

I agree with the previous post, no reason not to buy ahead with the flexible ticket. Are you planning the one day trip from Dublin to Belfast after the RS tour or on one of the Dublin tour days? We took the train to Belfast after the RS Ireland tour, then stayed two nights in Belfast. If you are going for the day an earlier train there and later train back would give you more time. Either way you have the option of a flexible ticket and a refund/change option. The Dublin train station was very close to our RS tour hotel.

thank you for the replies. we are planning to do it day 6 of the 8-day tour (weds). we already have flights booked for after tour so unfortunately very restricted. we will wait to purchase when we get there. super short window as tour ends noon weds, so 13:20pm Connolly and 20:05pm Lanyon Pl coming back.

Have fun and glad you have a window to go to Belfast!

Remember you can also take a bus.