Where is a convenient place to stay if we are taking the 8:20 ferry to Wales from Dublin port. It looks like there is a coach service from Dublin Center.
Then when we return to Ireland we may come back to Dunlaoghaire Port instead. We would arrive at about 1 PM and then fly out of Dublin the next day. Would it be best to stay in Dunlaoghaire or Dublin for that short time?

The port area is something of a wasteland of industrial units and docks.

I'd stay in a nice hotel in Dublin centre, and take a taxi next morning, around €18, 30 minutes tops.

Glenageary in Dun Laoghaire is a nice place to stay... it is a nice little town on the port, and plenty of B&B's are within walking distance to the DART, if you want to go to Dublin.

How could we get to the airport from Glenageary? This might be a nice option when we come back from Wales?

Dun Laoghaire is not at all unpleasant. If you want a bit of comfort, you could try the Royal Marine Hotel: http://www.royalmarine.ie/en/. There is a direct coach service linking it to Dublin Airport: http://www.aircoach.ie/fares/route-7...greystonesbray. If you prefer to look for more modestly-priced accommodation, you can still use the Aircoach service.

As #1 might be the only time we agree this year but we tend to stay in the Maldron Smithfield before heading to the Adventurer. Depends where you are aiming in Wales I suppose but don't discount taking the 21.15 crossing the night before and staying in the Travelodge in Holyhead for an early start.

The Gibson hotel near the O2 arena would be very close to Dublin port; however Dublin port is really central so you could stay anywhere in town.

In Dun Laoghaire the Royal Marine is probably the best as mentioned earlier but it can be pricey for what it is. There is another hotel, the Kingston Hotel, which is a good bit cheaper - it is a bit Fawlty Towers but has a nice Italian restaurant and lovely sea views from the beer garden.

Tony2phones, if we did the 21:15 ferry is it easy to reach the travel lodge as we can. to pick up a car until Monday morning, seems they are closed Saturdays and Sundays .

The Travelodge is 10 minute walk or quick taxi ride from the terminal. Rental cars are not common at Holyhead most people have their own transport or Rail and Sail, Hertz are open 8am till 2pm on Saturday at the terminal and offer their 24/7 service at that location.(ie there will be a car on the car park to which they give you an access code). Europcar offer a meet and greet service which is basically 24/7.