These are the kinds of details that reward a slow walk around St. Mary's Church on the hillside above Howth harbour, a ruined medieval structure that tends to get overshadowed by the headland's more obvious attractions, yet repays close attention in its stonework alone.

The site has a long and layered history. The original church here was founded around 1042 by Sitric, King of Dublin, though what visitors see today is largely the product of fourteenth-century building work, with various later additions and alterations. The double-aisled plan, divided by an arcade of pointed arches, is built of coursed masonry, and the two aisles differ slightly in length, the north running to about 20.7 metres and the south to 23 metres, while both are close to six metres wide. The western gable was widened at some stage to carry a triple bellcote, a small turret-like structure designed to hold bells, and an external recess with a high semi-circular arched opening was added to the same gable. Windows vary considerably across the building: the north aisle has a double-light tracery window to the west and a triple-light window with semi-elliptical heads to the east, while the south aisle's eastern window carries cusped ogee and trefoil heads, carved details that speak to changing decorative fashions across the medieval period. Inside, there are two piscinas, shallow stone basins used for rinsing liturgical vessels during Mass, as well as a stoup set into a trefoil-headed niche in the south wall. In the southwest corner of the south aisle, a chantry chapel contains the tomb of Christopher St Lawrence and his wife Anna Plunkett.

The church sits on the north-facing slope of Howth Hill, with views across the harbour and out to Ireland's Eye, the small island just offshore. Access is straightforward from Howth village, and the site is classified as a National Monument. The roofless shell means weather matters; the carved window tracery and tomb details are best examined on a dry day when the light is reasonable. It is worth moving slowly through both aisles and looking carefully at the variation in doorways, some sandstone, some dressed limestone, some with plain pointed heads of uncertain date, as the building's long sequence of alterations becomes legible in those small differences.