What is the best way to get to Conwy, Wales, flying from USA.

I've visited Conwy Castle twice...and while it is charming for a couple of hours...I'd have to wonder if you want to take all the time necessary to visit it, unless you plan to take the ferry from Holyhead. Spend a few days to visit the other King Edwards Welsh Castles to make it really memorable. There are train lines from London or Manchester which has the closest major international airport. If there are 9 in your group maybe you should rent a van. Taking a vehicle on the ferry can be expensive...so I would check return location options. There is also a ferry from Liverpool.

Fly into Manchester. It's a couple of hours away by train, an hour and a half if you drive.

I've seen Ryan livery at MAN.

Manchester would be the most convenient airport to land in the UK. From here, you can get by train to Conway (English spelling) / Conwy (Welsh spelling) in about 2 hours 30 minutes. Check out the trains at www.nationalrail.co.uk From MIA to CNW.Click DETAILS to find change points. If you do have to land in London, your departure station will be Euston (EUS). If you just show up on the day and buy tickets, it will cost you dearly for the journey from London. So, if this is to be your route, pre-book (up to 3 months in advance for cheapest fares) your specific train at www.virgintrains.co.uk Virgin Trains from London do not stop in Conwy but they do at nearby Llandudno Junction (LLJ). Please inform the Arriva Trains Wales conductor (if you are on one of their trains) that you wish to get off at Conway as it is a request stop.
You may be able to get a group discount for your fare. (You should also negotiate a good discount directly with any hotel that you plan to stay. I would not bother with agents as they will just want to take a good cut).

You can easily go by train & ship from Conway to Dublin (via Holyhead). You can buy a through train & ship ticket here:>http://www.arrivatrainswales.co.uk/SailRail/

You can find bus routes here: > http://www.transportdirect.info/Web2/Home.aspx?&repeatingloop=Y

Conway is on the doorstep of the Snowdonia National Park and the area is well worth seeing. You may wish to consider hiring a mini coach & driver for a tour of the Snowdonia National Park. (I would favour going to Portmeirion via the Aberglaslyn Pass with a return via another route). If landing in Manchester, the same hired mini coach could perhaps pick you up so consider doing a multi-day deal as you are a party of 9. A quick Google turned up this local company > http://www.walescoachhire.co.uk/coach-hire-wales/item/337-llandudno-junction-coach-hire-conwy (click minibuses).

Conway is still surrounded by the original medieval walls and the streets are narrow with hardly any parking. When I was last in the area, I paused briefly at this place - which is just across the estuary from Conway and mid-way between Conway & Llandudno.
http://www.quayhotel.co.uk
http://www.visitllandudno.org.uk
It has excellent reviews and would be easy for someone with a mini bus to pick you up & drop you off. If you travel by train, Deganwy station is about a 10 minute walk (on the level) from the hotel and is on the line mid-way between Llandudno Junction & Llandudno. (Look at Google Earth to see where these places are).
//www.visitsnowdonia.info
http://www.portmeirion-village.com
http://cadw.wales.gov.uk/daysout/caernarfon-castle/?lang=en
http://www.snowdonrailway.co.ukhttp:
//www.visitwales.com
http://www.bodnantgarden.co.uk

There's been a bit of editing of the first post since the two following responses. But it still has some problems.

As I understand it you want to hit Conwy first. Therefore ferry discussions from Ireland have no bearing on the subject. Additionally, you can't take a rental vehicle across the Irish Sea or it voids the insurance. Ferries and trains will take forever.

Conwy is a fair castle and championed / highlighted by Mr Seves, but there's no way I could squish it into the top five in the Principality -- and I've seen them all a bunch of times.

However, if you're going to keep trucking down the road, what I'd do is fly multi-city from home to Manchester and then Dublin to home -- filling in the gap with a cheapo Ryan-Dublin flight.

Now the ground transportation problem:

. The train station at MAN is located amongst the terminals. There's going to be a change somewhere, but I can't visualize where it would be. The walk from the Conwy station up to the castle is easy. If you decide to contine to the RoI by ferry from Holyhead, you can catch the train from Conwy, but I don't know if there's a change around Bangor to get across the Menai and on up to the ferry port.

. Driving out of the Manchester airport is a no-brainier. Coming back in is a bear unless you've done it a few times. There's no car return signs until forever -- what you do is follow the signs to T3 and then press on until you see you see signs for Car Rental Village. I just did it a half hour ago (and not for the first time by a long shot) and I can't start to recall how many compound roundabouts you have to transit to get through the maze.

. A van that holds nine is the largest you can rent without a cdl. They're not readilly available and, when all seats are in use, have no residual luggage space. I'd intended to snoop around on the arrivals level and at The Station but was running out of time and desperate for expresso. You lost, sorry.

Thank you all so very much. You have been very helpful!

Ed suggests Ryan Air - beware that they can charge a LOT for putting baggage in hold. Crossing the Irish Sea from Holyhead on a fast ferry can take under 2 hours = the standby time that you have to be in an airport. Throw in the fact that you can easily (& cheaply) take trains to Holyhead from either Llandudno Junction or Conwy & I would go for the ferry. (Ferries can be delayed by storms). Here are the sites of the ferry operators.
http://www.irishferries.com/uk-en/routes/dublin-holyhead/
http://www.stenaline.co.uk/routes/holyhead-dublin