Rosslare Harbour

If you're talking about the port itself, there's no overnight parking, there's a pub in Kilrane you can stay in, just ask and have a beer or two or a meal,

From the BF Website. “ The car park inside the port is operated by APCOA and has 335 spaces.

Charges are €1 per hour, €5 per day and €25 per week.

Payment can be in cash or by credit or debit card.”

I assume that since you can park for a week you can park overnight. At Portsmouth you can overnight in the check-in lanes but you have to arrive very late. Whether that’s possible at Rosslare I don’t know, but a Tweet to them asking the question will get a reply.

Thank's for all the replies, we arrived at 0.30am and stayed over in the Super Valu carpark, plenty of room and quiet.
We entered the port at 0745am and saw no vans parked in the carpark.
Just an aside, the Salamanca ferry no longer has a microwave in the passenger lounge, thankfully we had our travel kettle, pot noodles and porridge with us, so didn't have to buy any food on board.

I have to disagree about the quality of the food onboard Santona/Salamanca in the Azul restaurant. We go to Spain for 3 months in winter, and rarely eat out, so it’s a treat for us to dine on board. The buffet is superb and you can have as much as you like and the mains are also excellent. And I don’t think the prices are that unreasonable.

Oxymoron if ever I saw one.

properly cooked well done steak

But hey each to their own.

That buffet is in the club lounge and at €95 a head, in my opinion it's certainly not good value where Chrildren are concerned. It probably of value on the Bilbo / Santander sailing, especially if you're on the two night trip.
As usually the reviews are worse than useless, as food, service and value, is either Sh1t or brilliant.
So I would advise try it yourself. Speaking as someone who last used the food service on a Brittany Ferries boat, 30 years ago, it was great, 4 big people ate in the restaurant, 5 sort of little ones ate in the Cafe and joined us for to have our dessert.
I stopped eating on Irish Ferries when the stopped allowing us drink our own wine. Generally now we have a nice lunch before we board, in Brasserie Marcel's beside WBS, and bring a cool box, of food and drink, for later and breakfast.
I always tell them why on the after trip survey, (they usually send,) they are not entitled to Michelin star prices, and they are in a drink duty free arena, even recommending that they give the Francis to SuperMacs or Mac Donald's,