Her older sister, Madison, played college soccer at Rutgers before becoming a full-time assistant coach there during Riley's sophomore season.
In the NCAA tournament, she had four goals and two assists, including one of each in their 2–2 quarterfinal draw (and shootout win) against Alabama to send Rutgers to the national semifinals for the second time in program history, where they lost to Florida State.
She returned to a double-digit points total in her senior season in 2024, scoring 4 goals and providing 11 assists in 18 games, and was named third-team All-Big Ten.
She had two assists during the Big Ten tournament as Rutgers reached the final, but she was suspended for that game after picking up a red card in the semifinal against USC.
Angel City announced that they had signed Tiernan to her first professional contract on March 3, 2025, on a two-year deal with a club option for another year.