Backpacking holiday in September

You can use the National Rail planner to find trains serving your destination:

http://ojp.nationalrail.co.uk/personal/home/search

There are direct trains from Holyhead to Prestatyn, taking a bit over an hour. There do not appear to be trains serving Dolgellau, but there may well be a bus, or maybe a train that gets you close.

As for which is the better option, starting in Prestatyn vs. doing the mid-section, have youmread descriptions and reviews of the trail itself? I have no personal experience other then walking a short section of the trail near Abergavenny as part of the Three Castles walk. It was muddy! (And looking at the trail website, I see a detour near the southern end of the trail due to mud).

When you say "backpacking", are you planning to carry camping gear and camp along the way? Campsites are listed on the trail website:

https://www.nationaltrail.co.uk/offas-dyke-path

You will want good rain gear and waterproof boots or hiking shoes.

Thanks!

No, I'm not bringing camping equipment with me, although I dont mind purchasing if needed. Im relying on pubs/B&Bs for accomodation.

And waterproof map case for your OS Explorer Series maps.

OS Explorer Series:
1:25,000 Scale 4 cm = 1 km topographic maps with foot paths.
https://www.ordnancesurvey.co.uk/support/mapshop/what-map-shall-buy.html
The maps show public rights of way such as footpaths, bridleways and trails, boundaries, landscape features and places of interest.

For pub stays, I highly recommend the Hunters Moon which is practically right on the path, in Llangattock Lingoed ( near Abergavenny). We did not stay there ( we had a farmstay apartment nearby) but ate dinner there three nights in a row. Great food and warm hospitality.

There were quite a few answers on your previous topic from earlier this month

https://community.ricksteves.com/travel-forum/england/planning-a-4-weeks-trip-of-backpacking-through-the-uk

Have you shortened the trip?

I have :)
It has all been helpful, but I've got 1 question left unanswered

Whats the best option for me to enjoy both Snowdonia and Offas dyke within 2 weeks?

One possibility that works with public transport is to book a 4-day walking holiday ( guided or self-guided) near Dolgellau with HF Holidays:

https://www.hfholidays.co.uk/holidays-and-tours/self-guided-walking-dolgellau/

Their rates are all-inclusive (nice accommodations with private bath, excellent food, and transport to trailhead if you chose guided walking). The website gives clear directions for reaching Dolgellau and the country house. The walks are led by trained volunteers at three levels each day. The camaraderie we have experienced on these holidays is a major benefit.

Then make your way to an appropriate starting point for nine days on Offa's Dyke path.

Use this website to find public transport in Wales:> www.travelinecymru.info

Trains run very other hour from Porthmadog to Harlech > Barmouth > Aberdyfi (Aberdovey) > Machynllleth > Newtown > Welshpool > Shrewsbury . Birmingham > Birmingham International (Airport). So, you could go on the ferry from Dublin to Holyhead, then bus or train to Bangor and another bus to Caernarfon or into the heart of Snowdonia - such as Llanberis or Beddgelert for a base for hiking the mountains. When done with that area, train from say Porthmadog to Welshpool to do part of Offas Dyke.