The earliest register of baptisms (1812-1837) is at the PRO in London,
but I believe there is a transcription available on the net (see GENUKI).
Minute books, other baptism registers and collection books for 1812-2002
are at the Anglesey Record Office in Llangefni.
Bangor University Archive has the chapel minutes for 1882-1891.
Gwynedd Family History Society has transcribed the memorial inscriptions.
As far as I can tell the chapel is still going strong. And so is the burial ground -
my second cousin was buried there last winter. The bulk of the older records
is probably now at the Anglesey Record Office.