The 20-year-old is a youth international for the Republic of Ireland and moved to English football in 2023 from Cork City, costing the Seagulls £50,000.
After a short stint with the Academy, O’Mahony has scored one goal in four appearances on the South Coast before joining newly-promoted Portsmouth on loan last summer.
Suffering with injuries which limited his game time, he still managed three goals in 13 appearances for John Mousinho’s side, who survived comfortably in their first season back in the second tier.
Currently without a recognised striker, with Jayden Wareham moving to Exeter City last month, O’Mahony is expected to be the third loan signing of the summer after Matty Jacob joined for 2025/26 from Hull City and Finley Burns joined from Manchester City just 15 minutes ago.
CEO, Joe Jacobson, said: “Mark is someone who has been highly sought after in this transfer window. The Manager speaking to him personally has made him want to come here, Noel’s thoughts on the player and how he sees him playing and developing was key.
“It’s an exciting addition, and another player with experience in the league above. He’s very highly thought of at Brighton, and we’d like to place on record our thanks to them for entrusting us to offer him game time and provide further development.”
First Team Manager, Noel Hunt, added: “Mark is a young hungry striker who wants to come in and develop at our club. He’s young and he wants to learn his trade after having a successful loan deal last year at Portsmouth in the Championship.
“He learnt plenty whilst playing at that level, which will stand him in good stead for the upcoming season. He’s another player who has come through a really strong Academy, and will be a great addition to our squad.”
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