Eigg's Wikipedia page has a lot of information on the island.
The given co-ords are NOT where the cache is hidden; they point to the trig-point high atop An Sgurr, which has been a Ye Ole Survey Monuments (GC45CC) location in the past and remains loggable. In order to find the hidden 'Eigg Box', you'll need to follow the clues below...
The hum of hydro, the whirr of windmills
This is a modern place, shy of diesel bills.
They used to rely on generators but now
The smelliest thing on Eigg is a cow.
By the harbour you'll find information
On the main A00kW watery power station.
Also close, a standing stone.
A marker of this as Home.
For the folks who bought the island, fair.
To secure their place, right there.
They made this their slice of heaven
Back on June 1B, 1997.
An Sgurr is a striking rock.
Fans of Close Encounters may be reminded a lot
Of Devil's Tower in Wyoming
And be tempted to go a-roaming.
It's safe enough in good weather
To climb this C93m treasure!
(Any OS Map will give you the figure,
Or divide 1,289ft by 3.28, with vigour!)
Now follow purple dots to your prize
Somewhere dark, you'll find it lies.
So take care and bring a good light
To stop this one putting up a fight.
The box you'll locate beyond a hole
Below N56° 52.(B+B)BC W006° 08.(C+C+A)CA - Think Mole!
This spot has some history, of grizzly murder no less.
So for -perhaps supernaturally- chilly conditions dress.
Huge thanks to the Eigg Heritage Trust for permission to hide this cache, and to Maggie Fyffe (Trust Secretary) and the community on Eigg for their help in maintaining it. I hope it brings extra visitors to their remarkable island. Be sure to reach the 'singing sands' in the north of the isle, where the quartz sands squeak as you walk on them.
Other caches in the Small Isles series: Rum, Canna and Muck. And as of August 2015, another Eigg cache.