Ty'n-yr-Allt farm is located about quarter of a mile north west of the small village of Seion, and a mile and a half east of Y Felinheli. According to Gwynedd Archaeological Trust, Ty'n-yr-Allt was one of the farms which 'formed part of the core of the Vaynol estate from its earliest times.' On an estate rental of the year 1696, Ty'n-yr-Allt was known as Pen yr Hwylfa, and was rented for £6 by Ellis Morris. By 1844, the farm had acquired the name Ty'n-yr-Allt, and a rental map of the time showed that it was 'occupied by William Jones paying £17 15s in rent.' The first edition of the 25 inch Ordnance Survey map of 1889 shows that 'there was some amalgamation of the fields belonging to Tynrallt, although this was limited. No discernible changes were noted on the 2nd edition map of 1900 or the 3rd edition map of 1914.' By 1925, the farms in the area around Ty'n-yr-Allt 'had been rationalised into three larger holdings: Fachell, tenanted by Thomas Parry and paying £16 10s for a half year rent, Tynrallt, tenanted by T. G. Hughes and paying £2 11s, and Tyn y Coed, tenanted by the representatives of Owen B. Thomas paying £24 3s. This indicates that the rationalisation, presumably to increase profits, took place between 1867 and 1925. In 1967 these properties, incorporating the entire study area, were sold in the major Vaynol estate sale of 28th July 1967, when the estate was substantially broken up and the farms sold into private hands. "Tynnallt" farm is described at this time as having 37 acres of permanent pasture, 10.5 acres of arable, 7.5 of rough pasture, and 18 of rough grazing.'
Source: Gwynedd Archaeological Trust report entitled 'Seion Waste Water Treatment Works, Seion Gwynedd: Archaeological Evaluation,' July 2018 (Project no. G2563 and Report no. 1435)
M. Ryder, RCAHMW, 11th April 2019