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.

It forms part of the City and County of Cardiff and is now managed by Cardiff Council's Flat Holm Project Team and designated as a Local Nature Reserve, Site of Special Scientific Interest and a Special Protection Area, because of the maritime grassland and rare plants such as rock sea-lavender (Limonium binervosum) and wild leek (Allium ampeloprasum).

[4] In June 1815, a Dr Thomas Turner visited Flat Holm in a small boat and was stranded for a week due to high winds.

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.

[12] 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.

Property records from 1542 show that King Henry VIII granted a lease to farm the island to a gentleman by the name of Edmund Tournor.

It has been alleged that an old mine shaft on the north side of the island connects with a series of natural tunnels, and a concealed exit to the sea.

[14] Although Flat Holm was in full view of both the Welsh and English coasts, customs authorities were powerless to act as they had no boat to take them to the island.

According to tradition, a small cave in the east cliff at Flat Holm was used for the storage of contraband, mainly tea and brandy.

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.

[17] The island made communication history again on 8 October 2002, by becoming one of the first areas of South Wales to link to the Internet through a wireless connection deployed by Cardiff Council as part of the Flat Holm Project.

Cardiff Council's communication consultant Spencer Pearson made the first telephone call from the island 108 years after pioneer Marconi's wireless transmissions.

[19] On 23 October 1817, a British sloop, William and Mary, foundered after hitting the three rocky islets known as The Wolves which are not far from near Flat Holm.

It rises in a gentle slope from the exposed western rocky shore to more sheltered easterly cliffs, at the top of which stands the prominent lighthouse.

[34] There are argentiferous (i.e. silver-bearing) galena deposits on Flat Holm; the pits and mounds visible on the surface of the island are a result of trial borings.

[2] Records show that monks from St Augustine's Abbey in Bristol established a dairy farm and grange on the island after Flat Holm was granted to them by Robert, Earl of Gloucester in 1150.

He noted that the tax-exempt abbey farm was prospering and counted "seven cows, two bulls, five sheep, one horse, two pigs and two dogs".

Flat Holm was fortified in the 1860s following a visit by Queen Victoria and Prince Albert to France, where they had been concerned by the strength of the French Navy.

Nine Rifled muzzle loaders (RMLs) on disappearing Moncrieff carriages were built at the Royal Gun Foundry, Woolwich.

[40] The men were stationed on the island to keep the guns in good working order but the garrison would have been reinforced by regular artillery and volunteer troops if ever an invasion had been imminent.

[10][41] The structures formed part of the Fixed Defences, and protected the Atlantic shipping convoys between Cardiff, Barry, and Flat Holm.

[43] In 1896, the Marquis of Bute, then-owner of Flat Holm, agreed to lease all the land that was not already in use by the military or the lighthouse to the Cardiff Corporation for £50 per year.

The first engineer, Thomas Smith, and able seaman Robert Doran, were deported to the isolation hospital on Flat Holm to prevent spread of the disease.

[10] In March 1995, the county council agreed to obtain Flat Holm through a 50-year lease from the Crown Estate starting on 12 December 1995.

[51] In June 2021 Cardiff Council allocated £1.1 million (including £645,000 from the National Lottery Heritage Fund) to repair the Foghorn Station roof, stabilise and re-roof the Old Cholera Hospital, and create new guided tours of the island.

[46] Related to the onion, the leek has a bulb that grows for several years producing only leaves, then blooms with large purple flowers that smell of garlic.

[53] These include dove's-foot crane's-bill Geranium molle, an anodyne plant claimed by Nicholas Culpeper to have a wide range of medicinal uses and an "excellent good cure for those that have inward wounds, hurts, or bruises, both to stay the bleeding, to dissolve and expel the congealed blood, and to heal the parts, as also to cleanse and heal outward sores, ulcers and fistulas".

[54] The wild peony (Paeonia mascula) was introduced to the island (and nearby Steep Holm), possibly by monks,[55] and has naturalised.

One remaining plant was reintroduced by David Worall, the Flat Holm Warden, in 1982 and is protected by fencing near the path to the lighthouse.

The first, which was likely to run until late 2008, would reach an initial view on whether there are any fundamental issues that would preclude a tidal scheme in the Severn Estuary.