[John Mulcahy]

I can't help you too much (most of my research material is in storage), I believe this is more likely to be a pre-1871 item.

The 87th Regiment was long titled "The Royal Irish Fusiliers" and their button pre 1871-2 (when the general service button was introduced for other ranks) was the Eagle of the French 8th regiment. I can't tell if this is likely an officers or an other ranks button. If officers I would say pre-1881

The marking is unusual to me and does not to appear to be a maker's mark, I wonder if it is the owner?

As to why in Manning's Heath?, you would need to research the stations of the 87th (The United Services Magazine would have them) to see if they were stationed anywhere nearby. That is a tedious process and the station of the regiment may have nothing to do with the location of the find (consider the possibility of it being the residence of the owner, an item lost from a collection etc).

John