I was unable to find any reference, online, to this carving of a brooch on a rock in Saint Anne's Park which contains many follies. However, I suspect that it is a representation of the Brooch Of Tara.
The Tara Brooch is an Irish Celtic brooch dated back to the late-7th or early-8th century of the pseudo-penannular type (i.e. with a fully closed head or hoop), made from bronze, silver and gold, with a head formed from a circular ornate ring that is highly decorated on both sides. Its upper half is hollow while the lower half is solid with fused terminals. The brooch was constructed from numerous individually made pieces, and its front and reverse sides are equally decorated with around 50 inserted cast panels containing highly ornate filigree. The borders and terminals contain multiple panels holding multi-coloured studs, interlace patterns, filigree and Celtic spirals. The brooch is widely considered the most complex and ornate of its kind, and would have been commissioned to be worn as a fastener for the cloak of a high ranking cleric or as ceremonial insignia of high office for a High King of Ireland in Irish Early Medieval society.
The brooch was buried sometime during the 11th or 12th century, most likely to protect it from Viking and, later, Norman invaders. It lay undiscovered until around 1850. Despite its title, it was found not at the Hill of Tara, but on or near the beach around Bettystown on the coast of County Meath. The name by which it became known was chosen by its first commercial owner, the jeweller George Waterhouse, as a marketing ploy for selling copies during the height of the 19th century Celtic Revival. For this reason, many art historians describe it with inverted commas as the "Tara" brooch.
Its decoration and ornamentation is so detailed and minute that in parts it can only be fully seen using magnification, leading to one 19th century critic writing that it was "more like the work of fairies than of human beings". Art historians see only the contemporary Hunterston Brooch (c. 700 AD) as an equal in craftsmanship and design. The archaeologist Niamh Whitfield called it "the most ornate and intricate piece of medieval jewellery ever found in Ireland", while the NMI describes it as representing "the pinnacle of early medieval Irish metalworkers’ achievement". It was acquired by the Royal Irish Academy from Waterhouse in 1868, and transferred to the archaeological branch of the National Museum of Ireland (NMI), Dublin in 1890, where it remains. It was cleaned in the late-20th century by the British Museum.
The brooch was almost fully intact when discovered, but has sustained substantial losses since. Ten of the front inserts and three studs are now missing, while two more have lost their filigree. Comparison with 19th century photographs show that when found only one panel was missing.
Two wood engraving made in 1852 show it, according to Whitfield, "in near perfect condition" with the now missing filigree, studs, and additional interlace designs in place.