Plas-ym-Mhenros is a substantial Snowdonian house of C17th origin with C18th/C19th additions, including the agricultural ranges [ ] of the house. The core of the range is a house of Snowdonian plan-type with massive end chimney, moderately chanfered beams, including the partition beam for the twin outer rooms. A fireplace stair formerly led to the two first floor chambers, the inner chamber having a fireplace. C18th/C19th alterations (in more than one phase) included the addition of a parlour wing and service-rooms at the rear. The roof-trusses are of C18th type with lapped collars and the dormers have been renewed in the C19th. Plas-ym-Mhenrhos is noted in the Caernarvonshire Inventory, Vol. III: West (1964), mon. 1693, p. 89b.

A wooden panel is preserved in the house with the inscription IHS / 1678. The date inscription does not date the house; the Caernarvonshire Inventory suggests that it is the former cover of the font at Penrhos parish church, also dated 1678.

The farmbuildings (principally stable and barn) are of C19th date contemporary with the modernisation of the house. They appear to have included accommodation for servants in the part of the range nearest the house which retains the round-headed windows also present in the Victorian dormers of the house.

The house was sampled in 2011 as part of the North-west Wales Dendrochronology Project in partnership with RCAHMW/ The Oxford Dendrochronology Laboratory reports:

The floor timbers from this building produced a 195-year site chronology dated to the period 1413?1607, with one floor beam being derived from a tree felled in spring 1592, a second beam being from a tree felled after 1600, and a joist being from a tree felled in summer 1608. The floor was most likely constructed in 1608, or within a year or so after this date, using at least one stockpiled or re-used timber. Three principal rafters sampled from the roof appear to have been derived from a single parent tree. The 73-year ring-width series from this tree could not be dated, but the roof was thought to be later than the floor.

R.F. Suggett/RCAHMW/October 2011.

Commenting as part of the crowd-sourcing project Crwydro, Jamie Davies also noted the dendrochronology of the building, agreeing perfectly with the information presented above and noting the site 'Dating Old Welsh House' as a source of information.
(Source; Crwydro, 05.08.2014)