Lonely Light

Lonely Light is one of the Iron Islands. It is a minor outlying island, not one of the main seven.

The Iron Islands are actually formed in two clusters. The main group has about 30 islands in it, including seven major ones. The other two dozen or so are just considered administrative subdivisions of the main seven. 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 eight 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.