It was created by Kensho Tokumura, a student of Tatsuo Shimabuku. He went on to become a high dan in Gojuryu as well. In recent times, as I am informed, he has traveled around Okinawa collecting up weapons systems that might otherwise be forgotten, and he has created and continues to refine kata around each of them, and has created a ryu which he calls Tokushinryu. It is popular in Europe, Okinawa, and North America. In the US, Master Jeff Perkins leads the system. Our sensei is highly ranked in Tokushinryu, and several of our karateka in our dojo hold dan rank in Tokushinryu as well (as mentioned, I do not). Masters Tokumura, Anderson, and Perkins have been to our dojo to teach and promote.
So technically it is not affiliated with Isshinryu, but one of the organizations, WUIKA, has sort of informally embraced it.
There are a few Youtube videos with Master Tokumura demonstrating various kata. They may not be up-to-the-minute because he does continue to develop the kata and the system.