St Hilary's Church is situated in the centre of Llanilar, at the junction of several roads, including that of the A485 with the B4575. The churchyard is curvilinear and bounded by roads on its south and east sides, with its east side delineated by a stream, the Nant Adail.
The church was a parish church during the medieval period, belonging to the Deanery of Ultra-Aeron. Its benefice was a discharge vicarage in the patronage of the Bishop of St Davids. A ninth-tenth century carved stone, Llanfihangel Ystrad 2 (NPRN 275653) is now in the church porch. The stone is not in situ but was brought here from Cribyn, where it is thought to have originally been situated at Maes Mynach (NPRN 303883) or Y Gaer. The church is also home to a bell and oolite font from the ruined church of St Michael's Church, Rhostie (NPRN 100862). The font lies loose in the porch. The bell has been dated to 1350 and is inscribed '...Sc...A MARIA'.

The church is a Grade II* listed building, constructed of limestone rubble. It consists of five bayed nave and chancel, two-storey west tower, south porch, two-bayed vestry, bier house (north of east bay) and coal cellar (west of vestry). A medieval stoup lies loose in the church (although in 1810 it was noted in the porch, apparently in situ). The nave and chancel are thought to be thirteenth-fourteenth century in date and a lancet window in the east bay's north wall may also date from the fourteenth century. The gabled roof, with oak king-post trusses and cusped wind-braces, dates to the fifteenth century and is inscribed 'J.S. W.W.A. 1431'. The tower is thought to be fifteenth century and has a barrel-vaulted ground floor. The unusual heptagonal font bowl has framed facets and is thought to be of an artificial composite material. It has a plain oolite stem and baseand has been situated in the tower's ground floor since 1874. In 1810 there was a west gallery and carved rood screen. This had gone by 1833. There are blocked opposing doorways in north (blocked after 1856 and before 1874) and south (blocked before 1810) nave walls. The porch is thought to be post medieval in date and partially obscures the former south door. A window, blocked in 1874, is visible in the external east bay south wall. In 1870 a priest's door was noted. The church was restored in 1874, to the designs of R. Kyrke Penson, Ludlow. It was reseated and refenestrated, with new window and door-surrounds. The vestry/bier house was added and the west gallery removed. The font was moved and the church was refloored, with 35cm excavated beneath the church to facilitate the laying of concrete. The coal cellar was added in the twentieth century. The tower's west door was blocked in the 1930s.

Sources include:
Cambria Archaeology, 2000, Ceredigion Churches, gazetteer, 48
Edwards, N. 2007, A Corpus of Early Medieval Inscribed Stones and Stone Sculpture in Wales: Volume II South-West Wales

N Vousden, RCAHMW, 3 March 2014