Born in Erpeldange to a family of Portuguese heritage, the Luxembourgish midfielder was formed at FC Erpeldange 72 and the Racing FC Union Luxembourg academy, attracting interest from across Europe before joining Mainz 05 in 2016.

He signed his first professional contract in November 2018 and made his Bundesliga debut in February 2019, going on to spend five seasons as a first-team regular in Germany with more than 140 league appearances.

With his Mainz contract expiring, Barreiro joined Benfica in 2024 on a deal running to 2029, bringing energy, tactical maturity and box-to-box drive to the Encarnados' midfield. In January 2025 he scored the first hat-trick of his career in a 4-0 home win over Famalicão, and his honours at the club include the Taça da Liga in 2024-25 and the Supertaça Cândido de Oliveira in 2025.

A Luxembourg international since March 2018, when he debuted against Malta, he has become one of the most-capped players in his country's history and a leader of its national team.