Church and chapel data from The Religious census of 1851 : A Calendar of the returns relating to Wales, Vol 11, North Wales. Ed. by Ieuan Gwynedd Jones, UWP, 1981. The names given towards the end of each entry are those of the informants.
Parish statistics; Area 6915 acres; Population 237 males, 256 females, total 493
All the returns for this district (Festiniog) are missing
Rees, Thomas & John Thomas. Hanes Eglwysi Annibynnol Cymru (History of the Welsh Independent Churches), 4 volumes (published 1871+). Here is the entry from this book for Penrhyndeudreath chapel (in Welsh ) - with translation by Eleri Rowlands (June 2009)
LLANDECWYN (LLAN-DECWYN), a parish in the hundred of ARDUDWY, county of MERIONETH, NORTH WALES, 6 miles (N. E.) from Harlech, on the road to Festiniog, containing 462 inhabitants. This parish derives its name from the dedication of the church to St. Tecwyn : it is pleasantly situated in a part of the county abounding with lakes, of which Llyn Tecwyn Ucha, Llyn Tecwyn Isa, Llyn Eiddew Bach, Llyn Du, and Llyn y Dywarchen, are all within the parish. The lands, consisting of about four thousand acres, of which the greater portion is hilly, with a variety of soils, are for the greater part enclosed and in a good state of cultivation ; but there are considerable tracts of common. The appearance of the country is finely diversified, and the lakes materially contribute to the picturesque beauty of the scenery. Maes y Neuadd, in this parish, is an ancient seat of the family of Nanney. There is a lead mine in the parish, near the road from Tan y Bwlch to Tremadoc. The parish consists of one entire township and part of another, the rest of which is included in that of Llanvihangel y Traethau. The living is a perpetual curacy, with that of Llanvihangel y Traethau annexed, in the archdeaconry of Merioneth, and diocese of Bangor, endowed with £ 1000 parliamentary grant, and in the patronage of the Treasurer of Bangor Cathedral, to whose office is attached the rectory, rated in the king's books at £ 16. 14. 9 1/2. The church is an ancient structure, in the early style of English architecture, and is pleasantly situated on an eminence commanding an extensive view of the surrounding country. The Rev. John Jones, D. D., in 1719, bequeathed £ 50, the interest of which, according to the will of the testator, is applied to the instruction of ten poor children of this and the adjoining parish of Llanvihangel y Traethau. Mr. Richard Edwards, about the year 1764, bequeathed £40 ; and in 1769, Mr. Henry Poole bequeathed £5, the interest of which sums is annually distributed among the poor of this parish. The average annual expenditure for the support of the poor amounts to £ 134. 7. ( A Topographical Dictionary of Wales by Samuel Lewis, 1833)
Llandecwyn 'Revolt' School, c 1906 - on the People's Collection Wales site
"The 'Revolt School' was established in the vestry of Llandecwyn Wesleyan chapel in 1906, as a direct response to the Balfour Education Act which was passed four years previously in 1902........................"