The North Wales Gazette was the first english language newspaper in North Wales
and began at Bangor in 1808. Neither the British Library nor the National
Library of Wales has a full run, but there is a nearly complete run in the rare
newspaper section at Bangor University Library, which includes the issues you
requested.
There was a welsh language religious publication called Yr Eurgrawn Wesleyaidd
[The Wesleyan magazine] published in Dolgellau and in September 1820 it printed
a report on the ferry disaster which you can see in Google books:
http://books.google.co.uk/books?id=9yYwAAAAYAAJ&pg=RA1-PA70The first half is more or less the story reported in the Chester Courant but
the second half contains the names of the victims and I suspect this may have
come originally from the North Wales Gazette report.
I am including this list here as others may find it useful.
Robert Thomas and his wife (leaving 11 children), of the parish of Llanidan;
Catherine Williams, Ty'n y Pwll, Llanidan;
Elinor Williams, Trenant, Llanidan;
Margaret Williams, Llanidan;
Elinor Williams, Tan y Cowrt, Llanidan;
Hugh Jones, Tan y grisiau, Llanidan and his son;
Catherine Williams, of Lanidan;
Margaret Parry, Bryn gwyn, Llanidan;
Catherine Owen, Bryn gwyn, Llanidan;
Margaret Hughes, Trefan, Llanidan;
Edw. Humphreys, Trefan, Llanidan;
Ann Jones, Bryn Shenkin, Llanidan;
Jane Price, Bryn Shenkin, Llanidan;
Mrs. W. Lewis, Dinam;
W. Griffith and his wife, Rhos y meirch, near Llangefni;
John Hughes, Treauan, near Llanerchymedd;
Elizabeth Jones, Ty mawr, Llanddaniel;
Mary Williams, Glan y Rhos, Llanidan;
W. Williams, boatman;
John Evans, Llanddaniel;
A woman whose name is unknown.