[1] At the age of 14, Irina took part in a one-month tour promoted by the ITF with seven other girls and eight boys, joining the likes of Grigor Dimitrov and Ricardas Berankis.

At Wimbledon in 2007, she lost in the first round to Arantxa Rus; in doubles, she reached semifinal alongside Oksana Kalashnikova.

In October 2008, she won her first major ITF title at the $50k event in Jounieh, defeating Anastasiya Yakimova in the final.

At the 2009 French Open, she had her first major attempt to participate in the main draw, but failed in the final stage of the qualifications.

In July 2009, she made her WTA Tour debut at the Hungarian Open, but lost to Shahar Pe'er in the first round.

Later that year, she won three $25k titles and finished runner-up at the $50k event in Madrid and the $75k Toyota World Challenge.

Begu continued to progress at the ITF Circuit, winning her first $100k title at the Copa Bionaire in Cali in February, after defeating Laura Pous Tió in straight sets in the final.

In June, she returned to the ITF Circuit, where she reached the final of the $100k Open de Marseille, being defeated by Pauline Parmentier.

In July, she reached another WTA semifinal at the Palermo Ladies Open, losing to Sara Errani in straight sets.

At the 2013 Australian Open, she reached third round, alongside Niculescu, failing to repeat previous year quarterfinal result.

[5] In the first round, she defeated top 10 Angelique Kerber, but later failed to reach her first Grand Slam quarterfinal, losing to world No.

In September, she won WTA title at the Korea Open, defeating qualifier Aliaksandra Sasnovich in the final.

[15] At the French Open, she reached round of 16, after defeating two top 50 players, CoCo Vandeweghe and Annika Beck,[16] before losing to Shelby Rogers.

[17] In August, she won title at the Brasil Tennis Cup by defeating Tímea Babos in the final.

She then reached quarterfinal of the Premier-level Charleston Open, including win over top 20 Samantha Stosur, right before she later lost to Daria Kasatkina.

Nearly after that, she won title at the Bucharest Open, defeating top 50 Julia Görges in the final and not losing a single set in the entire tournament.

She followed this with her first 2017 French Open quarterfinal alongside Zheng Saisai, where they lost to Ashleigh Barty and Casey Dellacqua.

In July 2017, alongside Raluca Olaru, she won Bucharest Open, defeating Elise Mertens and Demi Schuurs in the final.

[29] She continued her good form in March and won a title on the WTA 125 Indian Wells Challenger.

[34] In September 2018, she reached the final of the Tashkent Open with Raluca Olaru, but they lost to Olga Danilović and Tamara Zidanšek.

[6] In February 2019, she and Niculescu won the title at the Thailand Open, beating Anna Blinkova and Wang Yafan in the final.

[5] She reached the semifinals with local favorite Sara Errani as a wildcard pair at the Italian Open, where they were defeated by Markéta Vondroušová and Kristina Mladenovic.

At the French Open, Begu reached the semifinals in doubles, partnering with Argentinian Nadia Podoroska for the second time in her career.

During the clay-court season, she reached the round of 16 in Charleston and qualified for the main draw in Madrid where she lost to Belinda Bencic.

[37] At the Palermo Ladies Open, Begu beat qualifier Marina Bassols Ribera, along with Océane Dodin, Diane Parry, Sara Sorribes Tormo, and Lucia Bronzetti, to win her first title in five years.

[38] In September, she won the WTA 125 2022 Țiriac Foundation Trophy, defeating Réka Luca Jani in the final.

128 using protected ranking, after a six months hiatus from the WTA Tour, she defeated wildcard Linda Fruhvirtová in the first round,[46] going out in round two against 18th seed Madison Keys[47] Also using protected ranking at the next WTA 1000, the 2024 Italian Open, Begu reached the fourth round with wins over qualifier Rebeka Masarova,[48] lucky loser Océane Dodin by retirement[49] and 27th seed Elise Mertens in straight sets,[50] before losing to 13th seed Danielle Collins.

[55] At the 2024 China Open she recorded a first round over wildcard Ye-Xin Ma,[56] before losing her next match to 17th seed Mirra Andreeva[57] As a result she returned to the top 100 in the WTA singles rankings.

[citation needed] Wins over wildcard Lucciana Pérez Alarcón,[58] Emiliana Arango,[59] eighth seed Anca Todoni[60] and qualifier Tina Smith[61] saw Begu reach the final at the 2024 WTA 125 Cali Open, where she defeated Veronika Erjavec to take the title.

[64] Partnering Monica Niculescu, Kostyuk represented Romania at the Paris Olympics, losing in the first round to Taiwanese pairing Hsieh Su-wei and Tsao Chia-yi.