No clue if it's pFAS free, but we bought a raincoat as a gift to bring home for my son in Killarney -- actually at Muckross House. Total tourist store. The said, brand was well reviewed and sale price better than buying in US (love google!). it was lighthouse UK - not Irish. We saw them everywhere (in stores and on people). We bought the other son an Icewear raincoat in Iceland that is amazing -- also cheaper in Iceland than in US and great quality. I bought myself a wax coat in Dublin -- jack Murphy off a sale rack near in the Aran Sweater market (also total tourist trap), but again cheaper than buying online at the time.
I could be wrong but I actually don't think you will spend more on a raincoat in Ireland -- I imagine the same cost or less. It's really whether you want to spend the time.
Otherwise, we managed luggage for 6 adults in Volvo SUV for 8 nights. We took two carry on size AWAY hard cases and three large Lands end duffle bag....squishable bags are key in a tight trunk. At least the four year old's clothing will be smaller so you can maximize their gear with extras if needed. We saw quite a few very happy, very muddy little kids along the way.