I've just been drawn into the story as I ordered a marriage certificate for my 3x great grandfather Hugh Thomas, who was Robert and Ellinor's son.
It's one of the more interesting (if awful) stories I've found in my research so far. I'm not having much luck locating original source material for the reports.
Funnily enough I was in Brynsiencyn a couple of weeks ago, just to have a quick look at it. I'm wishing now I'd done more prep before I went.
The National Library of Wales carries newspapers (free to read online). There is a very small piece in 'The Cambrian' (at the bottom of the page) dated 19 Aug 1820 regarding the accident.
Also, if you have a subscription that allows you to view the newspapers on Findmypast, there are several articles (pretty much the same information on all of them) in the following papers:
'Saunders's News-Letter', Dublin - 23 Aug 1820.
Sun (London) - 16 Aug 1820.
Chester Courant - 15 Aug 1820.
National Register (London) - 21 Aug 1820.
On another note, I have ancestors that lived in Brynsiencyn.