"Ian’s private hiding-place and look-out ever since he had been a very small boy" is the Pict House above the cove (GN16). The "shore party" find a biscuit box with his diary inside; the biscuit box had not been left by any ancient Pict as it has the trademark of a famous firm of Glasgow biscuit-makers. and Titty works out that the exercise book is in Gaelic and belongs to one of the natives. A savage Gael (GN4).
Ian is initially hostile to the crew of the Sea Bear and thinks that Mr Jemmerling is their ally. The shore party had pretended to be geologists and they bang one stone on another (GN5). Ian and Old Angus saw them disturbing the deer and want to ken who put them up to it if they return. According to Ian Deer are like salmon. They come back to the places where they begin their lives, and by shifting the hinds in the breeding season a dishonest man could ruin his neighbour's deer-forest and improve his own without laying hand on a single beast. It was the very meanest of tricks. And to use children for the driving made it meaner. Ian is described as a sandy-haired boy (GN16).
Later when Roger uses the Pict House as a look-out, Ian finds the "sentry" asleep there and plays a trick on him, labelling him THE SLEEPING BEAUTY (GN16).
But when the prisoners and the clan hear the two shots and Mr Jemmerling's destructive intentions become known, Ian and The McGinty help to protect the divers (GN26,27).
The tall old man is referred to as "the McGinty" and Ian is called "the young McGinty". The answer from outside their "prison" is There will be talk enough when the McGinty sees you. Chapter XXVI is called THE McGINTY LISTENS TO REASON (GN24,26,28)
However an anonymous Note makes clear that the name that they heard inside the "prison" was not McGinty, as if the real name was printed it would tell everytbody whi read it exactly where the Great Northern Divers had their nest .... at Dorothea's suggestion, the name McGinty was chosen, borrowed from Mrs McGinty whom Dick and Dorothea had met at Horning on the Norfolk Broads (GN24).