Cambrian Coast Rail

You might be better posting this specific question in the Wales forum.

"Is one direction better for this than the other?"

same-same

"Also how do you go about booking the train?"

www.nationalrail.co.uk

More information on train travel and tickets: https://www.seat61.com/UK-train-travel.htm

"And how far in advance does it need to be booked for travel in August?"

If you're travelling between Shrewsbury and Pwllheli the Anytime ticket costs 23 GBP, I don't think that breaks the bank. You don't even have to book online, just buy it at the station.

If that route is part of a longer trip from London, buy it in advance as soon as your travel date is fixed to grab the cheap fares.

See:>https://www.scenicrailbritain.com/lines/cambrian-lines

It is not the fastest rail route to the ferry port of Holyhead but it is more scenic than the northern route via Chester. The Cambrian Line also does not have a link from Porthmadog to Caernarfon and Bangor. So, I would suggest you travel from London (Euston) to the Cambrian Coast - you will need to change at Birmingham or Shrewsbury. You will also need to break the journey as it is too much to go all the way to Holyhead and then the ferry to Dublin in one go. Porthmadog would make a suitable place to break the journey. If you can spare a couple of days, you could also see Portmeirion. Anyway, you could go from Porthmadog to Caernarfon (castle) by bus but I would suggest you use the Welsh Highland Railway = www.festrail.co.uk

Then bus to Bangor and train to Holyhead for the ferry to Dublin. You could return via the north Wales Coast route and if doing so, I would recommend a night or 2 at Conwy or Llandudno. (See Conway Castle and if time Bodnant Gardens). The English border city of Chester is also well worth a visit.

Map of rail system:> http://www.nationalrail.co.uk/static/images/structure/css/nationalrailnetworkmap.pdf
For buses in Wales:> http://www.traveline.cymru
For through rail tickets to Ireland including ferry :> https://tfwrail.wales/ticket-types/sailrail
www.visitwales.co.uk
(Also read Wales forum).

One caution is that the Welsh Highland Railway is a narrow-gauge sightseeing line and I don't know what their policy about heavy/bulky luggage is, so I wouldn't use it as an actual means of transport.

The Cambrian Coast Rail may not allow a full size check on bag? How do I find this out? Because if not then I probably won't be able to use this rail line and would have to use different means, and less scenic, to actually get from London to Holyhead. I was just hoping to enjoy the journey over to Ireland moreso than just getting there quick.

The Cambrian Coast Line is a full size ‘normal’ railway and you can convey luggage on their trains. The Welsh Highland Railway - linking Porthmadog with Caernarfon is a narrow gauge steam railway. They are also able to convey baggage on their trains but it will possibly have to go in the guards compartment.

If you intend to go from London to Dublin in 1 day, you will need to take the direct train from Euston to Holyhead with no detours into scenic areas of Wales. www.virgintrains.co.uk operate the direct trains.

Going the more scenic way; London to Porthmadog alone is 6 hours 10 minutes.

Not sure about going all the way to Dublin in a day. But if we did it would be from the Cotswolds rather than from London. Will have a full RS planning map soon and plan on drawing out our route and getting as close as possible with drive times. Thinking I'll have a rental for the Cotswolds and then return to go trains route from Shrewsbury or Birmingham. Or could a rental be returned where the scenic rail begins? And in Great Britain trains tend to be a faster mode of travel right, aside from the old steam and scenic rails?

If you have a car rental in The Cotswolds, then I would continue to keep the car and drive all the way to north Wales.
You don’t say where your starting point in the Cotswolds. There are plenty of places of interest between the Cotswolds & N Wales and to see them would really require a 2 day trip.
I will now suggest a route that takes in some interesting places:>
Leave Chipping Campden & join A44 going towards Evesham - which you loop around on the by-pass. Then B4084 towards Pershore
& pass south of Worcester to rejoin A44 going towards Bromyard. (It is very bendy). As you approach Bromyard, look out for signs directing you R to LOWER BROCKHAMPTON HALL. It is a bit of a drive but at the bottom of the hill you will find the Hall. (Everything you expected Ye Olde England to be).http://www.nationaltrust.org.uk/brockhampton-estate/
Return to A44 & continue W to join A49 heading N. Do not take Ludlow by-pass but drive into this small town and take a look around.
http://www.ludlow.org.uk Just N of Ludlow - visit Stokesay Castle (route A49).

Then head either towards Shrewsbury and enter Wales at Chirk - see Pontcysyllte Aquaduct. Alternatively, from Stokesay Castle - head for Welshpool and visit Powis Castle. Then head west to Dolgellau & the Snowdonia National Park.

Have you given yourselves enough time to 'do' north Wales? Consider seeing Bodnant Gardens, Conwy Castle and -
PORTMEIRION.http://www.portmeirion-village.com
Alternatively, if the weather is clear, you may wish to prioritise going to Llanberis and up Snowdon via the mountain railway. You can then add Caernarvon Castle.
http://cadw.gov.wales/daysout/caernarfon-castle/?lang=en
http://www.greatorme.org.uk
https://www.snowdonrailway.co.uk

The trains on the main lines are indeed very fast but on the winding railways through the middle of Wales, they are very slow. That includes the Cambrian Coast Line. The trains have fixed windows - so you are not able to lower the window to take photos.
If you are relying on Rick Steves to give you a route from The Cotswolds to north Wales, they will probably send you on the main highways via Birmingham and then along the A55 North Wales Expressway. This will be like driving on Interstates and miss the main scenic areas.

Don't really have much time planned in Wales in general. Not for lack of wanting to, just lack of time and trying to prioritize. That's where I was hoping to just get a glimpse of Wales still by taking the scenic Cambrian rail, spend a night, see Caernarfon Castle and then on to Dublin and Ireland. My original trip plan was far too much driving, going from London all the way around Scotland, the Irelands, and Wales and returning to London in two weeks. So I am now revising the trip and am thinking of roughly a week in South Southeast England, and a week in Ireland. This with the leisure day and possibly the scenic route through Wales. I know it doesn't do it justice, but again on a holiday time restraint of two weeks.
Maybe if Holyhead has a car return I could entertain the idea of those drives you mentioned. Partially wanted to use the train system as well since it's always intrigued me, plus heard great things about the Cambrian Coast Rail.

Well, I did not realise that you were trying to cover so much ground in the time you have available. That said, if you wish to use the Cambrian Coast line, it is certainly viable to take an express from London (Euston) to Birmingham or Shrewsbury and then take the train that goes to the Cambrian Coast. I would make for Porthmadog on day 1 to stay overnight. Next day - Welsh Highland Railway to Caernarfon, then bus to Bangor and train to Holyhead for the ferry to Dublin.

If doing the above, a word of warning about fares. If you purchase a through ticket from EUS > PTM (Porthmadog) - it will cost you £85.20 no matter how far ahead you book or pay on the day. We can reduce that massively by booking specific trains ahead and splitting the ticket. So, for example, I see that a Virgin express leaves Euston at 10.23am on weekdays bound for Shrewsbury. By booking well ahead, you can get an advance ticket for this train at £16.50. (It arrives SHR at 12.50 on weekdays). A train for Porthmadog leaves Shrewsbury at 13.29 and arrives PTM at 16.53 and by booking ahead, you should be able to get a price of £13.50. So, the total now is £30. That’s a whopping £55.20 saved per person! (By doing such a journey, you will experience both express trains and the more slow scenic line through Wales).

So, you might wish to go to www.nationalrail.co.uk and check out the schedules & fares for your desired date. (The cheaper ‘Advance’ fares are not likely to be loaded more than about 11 weeks ahead). I would also advise that you take a packed lunch & drink.