The church was altered and refurbished during the 19th century, including an extensive rebuilding by Henry Kennedy, the architect for the Diocese of Bangor, in 1882.

Its conservation is specifically included in the aims of a Chester-based charity that promotes health and the arts in Anglesey and the north-west of England.

[2] It is built from rubble masonry with a slate roof, and part of a font thought to date from the 12th century has been reused as a water basin in the porch.

There was once a tradition of decorating the interior with paper garlands, although writers differ on whether this was to celebrate parishioners' weddings or to mark the death of unmarried women.

[2] Henry Kennedy, the architect of the Diocese of Bangor, oversaw a partial but extensive rebuilding in 1882, which included reconstruction of the east wall and the addition of the porch on the south side.

[8] The church is built from irregularly positioned pieces of rubble masonry, and Kennedy added red sandstone dressings in his 1882 work.

[2] The 1937 survey by the Royal Commission on Ancient and Historical Monuments in Wales and Monmouthshire also recorded the existence of an oak poor box, with iron straps, bearing the date of 1740, and a plain silver cup from about 1685.

The east window dates from the late 14th or the early 15th century, although it has been rebuilt, with three lights (vertical sections separated by mullions).

[2] John Jones, who was Dean of Bangor Cathedral from 1689 to 1727, was born at Plas Gwyn, and is commemorated with a stone tablet on the south wall of the chancel.

[2] The south window of the chapel has stained glass in memory of Claud Panton Vivian, of Plas Gwyn, who died at the age of 24 during the Second World War.

This tradition was noted in the 18th century, when a writer thought that they symbolised the "hymeneal union" (i.e. marriage) of parishioners, because the garlands each had a pair of hands in the centre.

He said that this "little edifice is more remarkable for its simplicity, and the beauty of the rural scene by which it is surrounded, than for any matters of antiquity or curiosity in its construction, or contained within its walls".

[14] The Welsh antiquarian Angharad Llwyd and the writer Samuel Lewis (both writing in the 19th century before the 1882 alterations) each described St Mary's as a "small neat edifice".

[4][15] Writing in 1847, the clergyman and antiquarian Harry Longueville Jones said that St Mary's was "remarkable for being in one of the sweetest spots in the isle of Anglesey"[16] The interior, he said, was "greatly blocked up with pews", but was in "excellent repair", with "a degree of neatness and comfort about it quite unusual in this district.

[17] In 2006, a guide to the churches of Anglesey noted that the red sandstone used in the windows and in the bellcote was showing signs of "severe weathering" in places.