On 3 December 2018, Brighton announced that Mandroiu had been released and had signed for League of Ireland Premier Division outfit Bohemians. He made his debut for "the Gypsies" on 15 February 2019 in a 1–0 home victory over Finn Harps. His first goal for the club came a week later, when he found the net away to UCD in a 2–0 win. Further goals would come against Derry City and St Patrick's Athletic, with Mandroiu earning numerous plaudits for his performances. He was nominated for the SWAI Player of the Month award for both April and May, but lost out to Bohemians teammate James Talbot and Dundalk's Sean Gannon respectively. Such was his fine form in his debut season, that Bohemians announced on 19 May that the young midfielder had been given a new contract at the club.[3]
Mandroiu signed for Bohemians' arch-rivals Shamrock Rovers in December 2020.[7] In June 2021, it emerged that Shamrock Rovers were ordered by the FAI to pay Bohemians a €20,000 compensation fee following a dispute over his transfer.[8] He played an important part in Rovers' league winning season in 2021 and half of the league winning season of 2022, scoring 22 goals over 53 league games.[9] His displays for Rovers in 2021 earned him a call-up to Stephen Kenny’s Ireland senior international side for the friendlies against Andorra and Hungary. [10]
After an instrumental start to the 2023–24 season, Mandroiu received a League One Player of the Month nomination after scoring two goals and three assists in the opening five games.[15] On 14 October 2023, he was sent off in a 1–0 defeat to Burton Albion. Following the match, manager Mark Kennedy warned the player that he was at a crossroads in his career and unless his approach to the game were to change he could become "just another statistic".[16] Following the end of the 2023–24 season the club confirmed that they were negotiating a new deal with him.[17] On 27 June 2024, manager Michael Skubala admitted that Mandroiu was likely to leave the club following a break-down in negotiations over a new contract.[18]
Return to Shamrock Rovers
On 19 August 2024, Mandroiu returned to Shamrock Rovers until the end of the season.[19]
He earned his first cap with the under-21s in a European Championship qualifier at home to Luxembourg, coming on as an 86th-minute replacement for Connor Ronan. The Republic of Ireland would win the game 3–0 in what was manager Stephen Kenny's first game in charge. Following this, Mandroiu was not included in Kenny's squad for the Toulon Tournament.
In September 2019, Mandroiu was selected by Kenny for Ireland's squad in upcoming qualifiers against Armenia and Sweden. He came on as a substitute against Armenia and started against Sweden, with Ireland beating Armenia 1–0 at home and Sweden 3–1 away. He was subsequently called into the squad for the October qualifying clashes against Italy and Iceland, although he remained benched for the entire match against the young Italians.
On 24 May 2021, Mandroiu received his first call up to the Republic of Ireland senior squad for the summer friendlies against Andorra and Hungary, but remained an unused substitute in both.[20]