Stuart, I went to the Library this afternoon and went on the Ancestry website and found out this info, which with all your research you are probably aware of anyway:-
William Elias AKA William ap Elias born 4th March 1707 or 1708 at Gefail Talhenbont
He was Baptised 3rd July 1707 or 1708 at Llanystymdwy and married Ann Williams at Llanfrothen on 19th January 1730.
He lived at Clynnog but the address only said Plas y Clynnog and later moved to Plas Y Glyn Llanfwrog where he died on 29th June 1787 and was buried there.
I found the info below on Google but again it doesn't give his exact address in Clynnog. I would think that the County Archives for Gwynedd at Caernarfon would have some info on him as he was so well known then.
ELIAS , WILLIAM ( 1708 - 1787 ), poet . According to David Thomas ( Dafydd Ddu Eryri , q.v.) , he hailed from Clynnog — Elias ap Richard of Talhenbont smithy was his father, says J. E. Griffith ( Pedigrees ). He is said to have started life as a shoemaker , and the list of subscribers to the Diddanwch Teuluaidd , 1763 , and an occasional note in the manuscripts (e.g. Wynnstay MSS. 7, 105, 131, etc.), confirm this. Later, he became a farmer and land agent to the Wynns of Glynllifon . He m. Ann Williams , 19 Jan. 1730 , as is shown by a letter in Wynnstay MS 7 , and ten of his children are mentioned in the pedigrees, among them the ‘ William Elias junior ’ whose name is found in Wynnstay MS. 7 and also in the Diddanwch Teuluaidd . He moved to Plas-y-glyn , Llanfwrog , Anglesey , in 1774 , d. there in 1787 , and was buried at Llanfwrog 2 July , at the age of 79. Elegies upon him were written by Dafydd Ellis of Holyhead and by Twm o'r Nant . He had been taught prosody by Owen Gruffydd (q.v.) — some of the latter's manuscripts, e.g. Llyfr Madryn ( N.L.W. MS. 799 ), had passed into his possession — and the elegy (a cywydd ) upon his old master, 6 Dec. 1730 , is his principal poem. Several of his lyrics and englynion are to be found in manuscripts which formerly belonged to him — e.g. N.L.W. MSS. 799, 7892 ; Wynnstay MSS. 6, 7 . He was on friendly terms with Michael Prichard (q.v.) , and there was correspondence between him and Goronwy Owen (q.v.) . [See further the article Williams , Hugh ( 1722? - 1779 ) .]