Aidan is first seen in January 1993 as a young, Irish, apprentice footballer for "Walford Town FC". When Arthur Fowler notices his talents, he offers to assist Aidan as his coach and keeper. Aidan then moves in with Arthur and his wife Pauline Fowler.
Aidan catches the attention of Mandy Salter and it isn't long before they begin a relationship, much to Arthur's concern, as he is aware of Mandy's past misdemeanours. Mandy is a bad influence on Aidan and after persuading him to skip practice, she gets him drunk on tequila and plans to seduce him, until Mark Fowler interrupts them. Whilst on his way home, Aidan (still extremely drunk) climbs on top of a parked car and falls off, severely injuring his leg. Aidan is out of action for a while, but his leg does recover; however, shortly after he is involved in another accident during training. As a result, Aidan can never play again and his promising career is over. Mandy and Ricky Butcher try cheering Aidan up by luring him into club-culture. After taking ecstasy, Aidan passes out in an ecstasy-induced coma and is hospitalised. He recovers from the incident but after his parents hear of his escapades, they force him to return to Ireland. Mandy is not willing to give her boyfriend up so easily; she follows him. Mandy was given a lift to the ferry port by Mark. As she did she stuck her fingers up to Albert Square and said "Stuff the lot of you".
However, just as everyone in Albert Square thought they had seen the back of Aidan and Mandy, Mandy persuades Aidan to return to England in June, and to return to Albert Square, Walford again, where they quickly become Walford's most hated residents.
Upon returning, Mandy and Aidan are homeless so they break into Pete Beale's vacant flat and attempt to claim squatters rights, which only lasts days before Grant and Phil Mitchell remove them. They now start sleeping on the streets and survive by begging, stealing prescription pads from Dr. Legg and selling them to drug-dealers, and stealing food whenever they can. Mandy turns to clipping (soliciting as a prostitute and then running away with the unsatisfied client's money), which ends when she is arrested and Aidan makes her promise to stop. They eventually take up residence in a rundown flat at number 5 Albert Square. They steal electricity from the neighbouring houses, causing a massive power cut but the electricity board put a stop to their scam. Meanwhile, Aidan and Mandy spend what little money they do have on alcohol and drugs. In October 1993, they both find employment at the First Til Last grocery store in Bridge Street but when the job falls through, they begin to go into decline. Soon after, Aidan hears that his grandmother has died but because they are unable to reach him, he only discovers the news on the day of her funeral. Aidan visits Ireland briefly with money his parents send him, but learns on his return that Richard Cole has started a smear campaign in his absence – wrongly accusing him and Mandy of being behind a series of robberies around the Square. Everyone begins to turn against them, Grant even roughs Aidan up when he accuses him of stealing from The Vic, and Aidan becomes increasingly ill and depressed. Things worsen when Richard buys the flat they are squatting in. When they refuse to leave, Richard breaks into their squat and evicts them days before Christmas.
Aidan receives a letter from his parents telling him that they are disowning him after learning of his recent behaviour. Mandy had found them a new squat in the boiler room at the top of a tower block on the other side of Walford, on the estate that Mandy grew up on. After reading that his parents are disowning him, Aidan's depression gets so bad that he attempts suicide on Christmas Day 1993 by contemplating jumping off the tower block. Mandy manages to stop him but he blames her for his misfortune. He says he is going back to Ireland for good, without her, and deserts her, returning to Ireland to reconcile with his parents. Mandy says "But I love you" as he walks away but he carries on walking away to head to the ferry port, never to see Mandy again. He was never seen or heard from in Walford again, and stayed in Ireland where he could lead a normal life, well away from Mandy.
Mandy is devastated to lose Aidan, and returns to Albert Square the same day, lodges with Pauline Fowler, but due to too many bad memories, she leaves Walford herself early in 1994, stealing Pauline's purse. Many people were pleased that troublemaking Mandy was gone from the area.
In 2011, Mandy returned to Albert Square after 17 years away and asked Ian Beale if he ever heard from Aidan and Ian said no.
In 2017, another character called Aidan was introduced, also from Ireland. His full name was Aidan Maguire. Ironically his surname is the same as the actor, Sean Maguire, who played Aidan Brosnan. Coincidence? So much so that some viewers on social media thought Aidan Maguire was a recast of the 1993 character Aidan Brosnan until others explained that it is another character with the same first name.