At some point Cisco Ramon created the ring and gave it to Laurel. Laurel used the ring to help Mia Queen regain her memories of the original multiverse.
Later a hooded figure used their own memory ring to restore John Diggle, Jr.'s memories of being Deathstroke.
When Abra Kadabra began having dreams of a wife and son that he never knew existed, he ran all the test that he could on himself and realized that he was suffering from neuro-temporal displacement. With this knowledge he went to The Flash Museum, found a broken down Martian memory restorer and fixed it, eventually using it on himself and getting his pre-crisis memories back.