No need to apologise 4th... How are you to find these things out if you don't ask!...
I've never booked online, but when you phone, they'll ask you to be there a little bit early, and they'll get you and your companion on board before the rest of the passengers, so you'll have no bother getting into the wheelchair space... And if by chance someone HAS boarded before you, and puts their luggage into the (clearly signed) designated Wheelchair User space, the staff are very good at getting them to move it pretty quickly!...
The staff member who gets the ramp, and gets you safely on board will get onto the train with you to check all's well...
You will find the staff on the train to be VERY friendly, and helpful...
Try not to worry... I'd be sure all will be in order...
Enjoy the trip...
God bless...
Darth...
P.S. If you PM me, I can give you a name for a person at Passenger Assistance in Irish Rail...