He discovered two Christian graves located close together in a field 23 m (75 ft) northeast of the island's present farmhouse; one grave had been opened and contained a male skeleton.
Flat Holm came within the parish boundary of St Mary's, one of Cardiff's two parish churches, and was kept as a hereditary property of the Norman Lords of Glamorgan.
A survey by archaeologist Howard Thomas in 1979 unearthed a number of medieval potsherds in the vicinity of the farmhouse and found evidence of continuous occupation of the island including middens containing numerous animal bones along with oyster and cockle shells.
The Anglo-Saxon Chronicle records that in 1067, Gytha Thorkelsdóttir, mother of Harold II, the last Anglo-Saxon king of England, stayed on the island before travelling to St Omer in France after the Norman conquest of England.
After the invasion, Lord Robert Fitzhamon, a cousin of William the Conqueror, formed the Shire of Glamorgan in Wales proper, with Cardiff Castle at the centre of his new domain.
) is a limestone island lying in the Bristol Channel approximately 6 km (4 mi) from Lavernock Point in the Vale of Glamorgan. The island has a long history of occupation, dating at least from Anglo-Saxon and Viking age.
Religious uses include visits by disciples of Saint Cadoc in the 6th century, and in 1835 it was the site of the foundation of the Bristol Channel Mission, which later became the Mission to Seafarers.
Here he met Welshman William Preece who was at that time Chief Engineer of the General Post Office and a major figure in the field.
Marconi and Preece erected a 34 m (112 ft) high transmitting mast on Flat Holm as well as a 30 m (98 ft) receiving mast at Lavernock Point. On 13 May, the mast at Lavernock was raised to 50 m (160 ft) and the signals were received clearly.
Because of frequent shipwrecks a lighthouse was built on the island, which was replaced by a Trinity House lighthouse in 1737.
Because of its strategic position on the approaches to Bristol and Cardiff a series of gun emplacements, known as Flat Holm Battery, were built in the 1860s as part of a line of defences, known as Palmerston Forts.
Inside the coffin were two skeletons which had been doused in lime, indicating that the occupants had probably died from a contagious disease. Reolice derives from an Irish word meaning churchyard or graveyard, alluding to the belief that the island had religious significance as a place of burial to people at the time.