Web booking question

[comcor]

Next month, I want to take a one-way trip from Dublin to Wexford. Ideally, I would prefer to leave from Pearse on the 17:41 train.

Booking this I have two problems

The first is that the system doesn't seem to like bookings that leave from Pearse. I see a message that reservations are not possible for all Pearse-Wexford trains.

I could get over that if I knew that a ticket booked from Connolly would get me through the barriers at Pearse.

The second is that the 17:41 (17:36 from Connolly) doesn't seem to be bookable anyway, providing the following error

Your selection cannot be purchased online. You have selected T (Ticket Only) for all parts of the journey.
To reserve a seat - a P (Premier/1st Class) or S (Standard) box must be ticked for at least one part of the journey

Now, as explained earlier, I just want a single on the 17:36. Does this mean that I either have to book on the later 18:30 train to get a reduced price single or book an unneeded return leg or pay the walk-up fee for the 17:36? What is that anyway? My experience has been that buying singles in the station for Inter City can be prohibitively expensive.