What is the point of somebody from Dublin crossing to Holyhead to return from Fishguard and then be driving north through places in Ireland with which they would already be familiar? Far better to go across to Holyhead, do Snowdonia and then drive south-eastward through Wales (A470 = very scenic) to stay in the Brecon area (1 hour N of Cardiff) or push on & stay on the edge of Cardiff. Then, having done SE Wales, head for Pembrokeshire for 2 or 3 nights before heading back up the west coast & back on the ferry from Holyhead. That way, you have done a complete 'circuit' of Wales seeing new places rather than part of the journey being in your homeland.
From Fishguard to Cardiff would be 2 hours of non-stop driving so I would rule that out as a day trip. Swansea as a city is just not worth bothering with. Much of the centre got flattened in the war and the re-build used horrible 1950's architecture. Yes, Swansea does have a lovely position on a bay & a couple of nice parks such as Clynne Gardens & Singleton botanic garden but really it just ain't worth a visit. The only reason to drive through Swansea would be to reach the stunning Gower peninsula just to the west of the city. Rhosilli is considered to be one of the World's best beaches but the peninsula has a whole load of attractive beaches.
If you want cheap accommodation, then you could consider www.travelodge.co.uk - certainly for Snowdonia. Caernarvon or Porthmadog would be good Travelodges to stay but Caernarvon only has on-street pay parking (I think) whilst Porthmadog has guaranteed free parking outside the door. www.visitwales.co.uk will have links to accommodation sites including camping grounds. (Nights in September can be quite cold). I would also consider B & B or Farmstays. www.farmstay.co.uk/wales
Whilst in Snowdonia, I would definitely visit Portmeirion.
On leaving Snowdonia, I suggest you take the A470 to SE Wales. The Brecon area could make a good base as it is very scenic & about 1 hour's drive from Cardiff. However, it would be pushing it to see everything in the Cardiff area in one day. In the city centre - you have loads of shops in Victorian arcades as well as modern malls + Cardiff Castle and The National Museum of Wales. Then, 1 mile south, is Cardiff Bay with waterside restaurants, the Wales Millennium Centre (Opera House/Concert Hall), Norwegian Church, Dr.Who centre. Around the edge of Cardiff is Caerphilly Castle, Castell Coch & St.Fagans Castle / Museum of Welsh Life. If staying in Cardiff, you might also like to take a day trip to Bath as it is only a 1 hour journey. (You might like to use the train as parking in Bath is 'tight'). Do not stay in a central Cardiff Travelodge = noisy clubs & no free parking. A couple of hotels are located near M4 J32 which would be handy for those with a car. (You could commute into central Cardiff by train from Taff's Well or Radyr station if you wished).
For those with Caravans / Campervans, a nice site exists in Pontcanna Fields = about 1 mile NW of Cardiff Castle.
If you have stayed in the Brecon area, it is avery pleasant drive - mainly on the A40 to Pembrokeshire. If you have stayed in Cardiff, you can reach Pembrokeshire by using the fast M4 & A48. However, you may wish to take a detour from either route to visit the National Showcaves of Wales - about 1 hours drive NW of Cardiff or 40 minutes W of Brecon.http://www.showcaves.co.uk
Also on the way to Pembrokeshire, you might like to detour to Carreg Cennen Castle near the village of Trapp. (Take first left going N form Caves - cross the mountain - then L at T junction & you can reach it via backroads - if you are really smart).
In Pembrokeshire - consider staying near Tenby as this is the most attractive resort. Stunning cliffs & Bays just S of Bosherston = 30 min drive W of Tenby.
On drive up W coast, Aberaeron is worth a look as is the coast road via Barmouth & Harlech.
Google the places I have mentioned see Google Earth.