Irish's new ticketing system is a bit up the creek. Although, to be fair it was my fault for not spotting it.
Booked a Family Ticket and added on an extra kid and adult ticket. After booking, realised I hadn't actually added on the extra Adult ticket so emailed the ticketing team who have replied to say no problems to add on and pay for another ticket but did I want that ticket to be in the Block with the Family Ticket or the (entirely different Block) with the single Child Ticket.
So despite being on the same booking a Family Ticket and an extra child ticket are split up in different blocks. Tres bizarre!