There are no hard concrete facts given about that particular hack (that I am aware of), other than it was performed, and notorious enough for Trinity to be linked to it simply by name. But we can deduce a conclusion from what we are presented with.
tl;dr: Trinity hacked the IRS database before being unplugged. She, like Neo, had a life inside the Matrix before being brought into the real world by Morpheus.
Disclaimer: Do note that what follows is my personal interpretation, and as such, may be different from other's interpretations (including your own).
Parallels between Neo and Trinity
In the first few scenes where Neo is in his apartment, we are shown that Morpheus is presented as a terrorist/computer whizz/hacker to the people living inside the Matrix. There's a lot of news stories and articles flashing by on Neo's screen as he sleeps on his desk.
The next thing we find out is that Neo is marketing his skills as a hacker. He sells a program to 'Choi', and goes to warn him:
Neo: If you get caught using that...
The scene with Trinity in the club happens (I'll come back to this), and later we learn that he has an office job, that just wasn't fufilling or stimulating enough to keep his mind on his tasks. We know this because we are shown his working conditions:
- His boss is overbearing and rules by fear (threatens Neo with losing his job if he is late again)
- His cubicle has high walls, is only big enough for one person, and the cubicle opposite is unoccupied (No work colleagues/friends to vent steam with)
- All the cubicles look exactly the same. (Menial job with no variety)
- He doesn't even have much in the way of personal belongings there, just what is in his bag.
So why is all this important? Because we've been shown that Neo is a bored computer whizz unhappy with his lot in life and sick of the status quo, who gets back at 'the man' by writing and selling hacks.
But even that is not enough. Which is why Neo was collating this data, trying to find out as much about Morpheus (and the Matrix) as possible.
"That's all well and good, but what has that got to do with Trinity?"
Because before Neo and Trinity met, Morpheus had his eye on Neo (as a potential 'One'), and tasked his crew to watch him closely. Trinity saw a lot of herself in Neo, which made her identify with him closely:
Cypher: You weren't supposed to relieve me.
Trinity: I know, but I felt like taking a shift.
Cypher: You like him, don't you? You like watching him.
Trinity: Don't be ridiculous.
So what parallels was Trinity drawing between herself and Neo? There's the obvious searching for Morpheus and the Matrix, Trinity confirms for us in the club scene that that's exactly what she was doing.
Trinity: I know why you're here, Neo. I know what you've been doing … why you hardly sleep, why you live alone, and why night after night, you sit at your computer. You're looking for him...It's the question that drives us, Neo. It's the question that brought you here. You know the question, just as I did.
Trinity used to be just like Neo, that she also was a bored computer whizz unhappy with her lot in life and sick the status quo. who gets back at 'the man' by... doing stuff.
What stuff, you ask? An educated guess would be hacking into the IRS database. But we can do better than an educated guess.
"It was a long time ago"
If you've ever had a drastic/traumatic change in your life, you'll know the feeling when you look back on your old life: it seems so long ago, and sometimes its almost like you're reliving someone else's life.
Script writers and novelists love this trope, the idea of a sudden (and usually traumatic) shift in someone's life, leaving them looking wearily back at their past selves, either with a sense of longing, or a sense of "how was I that dumb/innocent/oblivious?"
With Trinity and Neo (and us, the first time we watched the movie) the traumatic incident that changed them forever was their 'rebirth' into the real world, and subsequent explanations of the machines and the Matrix.
In fact, it completely destroyed their preconceptions of the world around them. One could argue that Trinity and Neo were oblivious to the reality of what the Matrix was, before being unplugged. So Trinity's line:
Trinity: That was a long time ago.
Is just her reminiscing on the "lost obliviousness" of her former self. A time where institutions like The IRS, Police etc were (in her mind) real entities, not constructs of a virtual world.
Trinity hacked the IRS database before being unplugged from the Matrix. Her reasons for why she picked the IRS are unknown, but we can deduce from her backstory that she was a bored computer whizz who -unhappy with the status quo-, decided to get back at 'the man' by hacking things.
We can also deduce that the IRS was one of her more notorious hacks, as simply by hearing her name, Neo was able to link her to that particular hack.