Friends have used this, and other ferry services between Ireland and the UK, and France and UK so we were forewarned 'how' to do this well.
We prebooked our reserved seating upstairs, and enjoyed great seats, power for our electronic devises and free wifi for the very fast trip across the pond to Dublin.
We also booked priority car service, so we were one of the first on/off the ferry which made a huge difference, tho they certainly pack the cars in! There was very little room to open doors to get in/out of the car. Not sure how older people, or parents with toddlers would manage.
The crossing was uneventful; good weather .... so all in all a pleasant experience tho I'm very glad we did it the way we did, as there was organised chaos below with people trying to find seats for their family group; people sleeping across 3 seats; sharing tables with card playing blokes while others were simply sitting with their kids on the floor.
As one person before me said, surely there's a seat per ticket?
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