Trethias booked her ticket for the Kerrygold Irish Oaks with victory at Naas on Wednesday evening.
The daughter of Invincible Spirit had finished third in a Leopardstown Group Three last time out, but stepped up on that effort in the Listed Irish Stallion Farms EBF Naas Oaks Trial.
Partnered by Shane Foley, Trethias (11-2) proved two and three-quarter lengths too good for favourite Search For A Song in the 10-furlong contest.
Winning trainer Jessica Harrington said: "Shane said she just needed that strong gallop. She just loved it and she'll stay further.
"She relaxed and did everything right today. Today is the first time she's put everything together.
"She got no run in Navan at all and everything went wrong, and then the last day probably things just didn't go right either.
"Today everything went right. She had a good break since her last run and we'll head now to the Oaks (on July 20).
"She's beautifully bred as she's out of a half-sister to Dar Re Mi. She'll make a beautiful broodmare.
"She's now finally done what I thought she could always do."