Spike island cross

"Really know very little of this find, when I was maybe 12 or so washing cobblestones at back of my grandmother's house in ballindangan near mitchelstown co cork came across gleaming from mud in between cobbles, have never shown beyond family. My father is quite a history enthusiast and speculates was made from a coin by an inmate one might think in and about the time inscribed, the chain is just something we had at home. Sorry no scale but is 3-4m in length. If interested please contact me. Hope photos are indicative."