The Iron Islands are actually formed in two clusters. The main group has about 30 islands in it, including seven major ones. There is also a small secondary cluster, however, a few days' sail to the west: it is composed of a dozen rocky outcroppings and atolls, of which Lonely Light is the only inhabited one.
In the books
Lonely Light is explicitly not considered the eighth major isle in the Iron Islands: the World of Ice and Fire sourcebook (2014) repeatedly states that there are "seven major islands" in the chain. It is a tiny rock, and if it were located in the main cluster it would just be counted among the other two dozen or so smaller insignificant islands that surround the main seven.
The World book does specify that there are seven primary islands, all the ones from the main cluster: while there are technically 30 "islands" most of them are so small that a man can walk across them in a single day - so administratively, they are just considered part of whichever of the main seven they are off the coast of (by comparison, the Isle of Wight could technically be said to be "an island in the British Isles", but wouldn't really be spoken of as in the same category as Great Britain and Ireland). The names of these 20 or so smaller islands haven't even been given. As for the smaller cluster farther west, it really isn't worth much mention: apart from Lonely Light itself, the other "dozen islands" are basically just glorified reefs and rocks, none of them inhabited except by seals and sea birds. Even Lonely Light itself isn't much to speak of, and would have been considered just one of the lesser 20 or so islands in the main chain if it weren't so far apart from all the others. The small island is surrounded by rookeries of seals and sea lions. Administratively, Lonely Light is apparently just considered part of Great Wyk, westernmost of the main islands (as well as the largest).
Lonely Light, as the name implies, is extremely isolated, and indeed is farther west in the Sunset Sea than any other land - making it the absolute western limit of the Known World. The maesters suspect that if a ship sails west for long enough it will eventually reach Yi Ti, but after thousands of years, even the ironborn only report endless ocean to their west.
Lonely Light actually has been mentioned in the main A Song of Ice and Fire novels, in the fourth book during the Kingsmoot. They are ruled over by House Farwynd of the Lonely Light - who aren't even a major House but a cadet branch of the main House Farwynd that lives on Great Wyk. Given how isolated they are from the main islands (and all other points of civilization like mainland Westeros), the other ironborn consider the inhabitants to Lonely Light to be the proverbial backwater country bumpkins (barely a step above House Codd). Living alone and isolated amidst such a vast stretch of sea is also suspected to have given them all ocean madness: when Lord Farwynd put forth his claim at Euron's Kingsmoot, he wildly claimed he would lead all the ironborn west across the Sunset Sea in a massive fleet to settle on a glorious new paradise land (his comments were waved off as insane, no one voted for him, and he quickly gave up).
House Farwynd of Lonely Light's heraldry consists of a black longship on a black cresting ocean, set against a red setting sun in an orange sky - referring to the fact that they are the farthest west House in all of Westeros, the sun setting in the western sky.
Lonely Light is said to be about eight days' sail west of Great Wyk. In contrast, Harlaw is about one day's sail from Pyke. While mentioned in the fourth novel, Lonely Light hasn't appeared on any of the maps included in the books, but was in the Lands of Ice and Fire map collection (2012): it is directly west from Great Wyk, and observing from the map, it is roughly as far west from Great Wyk as Great Wyk is from Seagard to the east; it is at a roughly equidistant point between Fair Isle in the Westerlands and Cape Kraken in the North.