The Ruins of Conflict are where the guardian deity of Melemele Island, Tapu Koko, can be found. In Pokémon Sun and Moon, during the denouement, after the player first enters the Hall of Fame, they visit the ruins with Lillie and challenge Tapu Koko to a battle. If the player defeats or flees from Tapu Koko, they may return to the ruins after the credits roll to challenge it again, without needing to enter the Hall of Fame again. In Ultra Sun and Ultra Moon, this does not occur, and Tapu Koko can only be encountered and battled during the post-game.
In short, no. The Ruins can only be accessed after you beat Kukui, unfortunately.