What makes it particularly odd is an architectural detail easy to miss amid the overgrowth: from the vaulted roof of the chancel, a round turret rises upward, a feature recorded in drawings by George Petrie around 1828 and by William Wakeman around 1843, when a substantial portion of it was still intact. The turret's purpose is not explained by its appearance alone, and the combination of vaulted chancel and rising tower in so compact a building gives the ruin an unusually layered quality for what is, at its core, a fairly small early medieval church.

The church was dedicated to St Nessan, a figure connected to a seventh-century illuminated manuscript known as the Garland of Howth, one of the early gospel books associated with the greater Howth peninsula. The building itself is pre-Norman in origin, constructed from coursed limestone blocks with small packing stones filling the gaps and large squared blocks, known as quoins, reinforcing the corners. The nave measures roughly 8.4 metres in length and 4.5 metres in width internally, with walls 0.8 metres thick, and is entered from the west through a semi-circular arched doorway whose arch springs from roughly-squared imposts, the horizontal blocks from which an arch rises. Two narrow slit openings with widely splayed internal embrasures light the nave from the north and south walls. The chancel, inset and aligned roughly east-north-east to west-south-west, has three windows, including a tall round-headed window in the east wall. In 1235, the church of St Nessan was formally moved, as an institution, from Ireland's Eye to Howth on the mainland. The building was heavily restored during the nineteenth century, which accounts for some of what visitors see today.

Ireland's Eye is accessible by small boat from Howth Harbour, with seasonal services typically running during the warmer months. The island has no facilities and no permanent population. The church sits on relatively low ground above the southern beach, so it is reachable without a long walk once you land. Vegetation has made the structure difficult to read clearly, particularly inside the nave and along the wall tops, so it rewards a slow look rather than a quick glance. The round turret above the chancel is the detail worth seeking out, best appreciated by stepping back far enough to see the full roofline in context.