The stone and a poem connected with it are mentioned in ‘Footprints of Early Man’ by Donald A. Mackenzie, 1909 (online at google books) but there’s not much mention of the source:
A standing stone 20 1/2 feet high and 6 1/2 feet broad, with a notch at one side near the top, is situated 80 feet above the sea-level and facing the Atlantic on the west coast of Lewis. It can be seen far out at sea, and it [..] may have been a landmark for the guidance of mariners. Seen from a distance it resembles a human hand. Its Gaelic name is “Stone of the Truiseal”, but what “Truiseal” means is not known. An old Gaelic poem asks the “great Truiseal”:
“Who were the people in thine age?”
but the stone gives a very vague answer, saying it merely “longs to follow the rest” (the ancients), and that it is fixed “on my elbow here in the west”.
I found this additional fragment of the poem at
bbc.co.uk/scotland/islandblogging/blogs/005132/archive/2006/08.shtml
“The Truiseal stone is reputed to have been a man in by-gone days, who had been turned to stone. A passer-by had heard the stone proclaim in sepulchral tones:
A Truisealach am I after the Fiann;
Long is my journey behind the others;
My elbow points to the west
And I am embedded to my oxters.”
Your oxters are your armpits! so the stone must be very big indeed.