Canna's Wikipedia page has a lot of information on the island.
The given co-ords are NOT where the cache is hidden. They point at the top of Carn a' Ghaill. At 210m above sea level, it's the island's highest point. In order to find the cache, you'll need to follow the clues below...
Landing at Canna
No longer needs a ladder.
Since 200A, when the quay was unlocked
You can just stroll ashore without wet socks.
It was August 2B when a King, Charles,
Helped give dry-feet-loving visitors smiles.
The Scottish Church's gate is worth a pause.
As well as the tower, before it's lit and soars.
But how many fish are upon the gate? Be sure to go inside.
They're worth counting twice - avoid a bruise to your pride.
Your number should be C
And C should not be three!
Exit right and then swing left to take a walk
To the post office and a place where it's good to talk.
It's not Deal or No Deal but you'll find a red box to open.
Inside, hopefully nothing's broken.
Here's a number to see,
It ends ...46D [Temp help until the box is refurbished. D=0].
With the Punishment Stone and Celtic Cross to see,
And wildlife from Basking Shark to common bee,
And a cute lighthouse and a rocky keep,
Take your time with cache to seek.
You'll find your prize near twisted columns
at N57° 0B.(C+D+1)(A+2)(A+B) W006° 29.(C/B)(D+2)(B+1) - Nothing rhymes with columns!
Huge thanks to the National Trust for Scotland for their permission to hide this cache, and to Stewart Conner, the property manager and Winnie MacKinnon, the island's ranger, for agreeing to keep an eye on the cache for me. After finding it, you can follow the rough path (take care!) to see a memorial - a poignant spot for contemplation.
Other caches in the Small Isles series: Rum, Eigg and Muck. Plus a 2nd Eigg Box.
Read more about The Historic Geocaches' campaign.