Did I horribly mispronounce your name?

No, it's Af-rick. Like "Africa" without the "a".

You worked at US investment bank Morgan Stanley for much of the 1980s and 90s, and were the first female managing director on the trading floor. How difficult was it to translate some of those experiences into fiction for your latest novel, On the Floor?

The technical challenge was how to bring the world of finance to life and make it accessible to anybody. It can seem obscure from the outside, but it's full of fantastic, human stories. The last thing I wanted was big baddie bankers and their trophy wives; but equally, I didn't want to get weighed down with technical details. I started a really early version in the 90s, while I was still working, but I had to grow up as a writer to do this one in a way that I thought worked.

The central character in the book is Geri – "the skirt among men" – who is the star employee in an investment bank in 1991. Both of you seem to have thrived in these male-dominated environments – what is the lesson?

The discussion about women in high-level positions often gets bogged down in this glass ceiling thing, and the idea that all men are bastards is just not useful or accurate. I'm sure it took me a couple more years [than men] to get promoted, and I'm sure, relatively, I was paid less, but the people who promoted me and pushed me to the top were men. It was a meritocracy.

How would you move the debate on?

I think women need to ask: at what point do you start to take responsibility for things that might not be working out? If you go into a newsagent's and you look at women's magazines – not just Heat, but right up to the glossies – they are full of women telling women how to live their lives: how often to have sex or what makeup to wear, or whether it's OK to cry in the office. I find it interesting, having been in the working world since the 80s, that women are still needing to be told how to behave.

At one point in On the Floor, Geri sets out a "Rule Book for Wannabe Female Bankers". One of these is: "Treat all female colleagues with total contempt." Is it your experience that women are not always supportive of one another at work?

There were very few women where I was, but I've never found any fantastic solidarity among women, particularly in the workplace. I'm not sure that women think that way. There's this idea that if you admit to being ambitious as a woman you're somehow unfeminine, unsexy or macho. As a TV interviewer said to me in Ireland recently: "You must have had balls." It's not very desirable. I once wrote an article about ambition, and the only woman I could find who admitted to being ambitious was Madonna.

When On the Floor was nominated for the Orange prize, there was a bit of a kerfuffle about it and the whole approach of the female journalists I spoke to was: "OK, you had a job that you hated, now you have a job you love, how do you feel about that?" The assumption being that I must have hated working in the City because it was a very unpleasant male-dominated environment. But I didn't, I loved it.

Is it true that when you started at Morgan Stanley you were asked if you planned to get married or have children?

Not in my official interviews when I started, but I made an internal transfer to the trading floor after a few months and I was interviewed by a guy who ran the trading division. At the end, he asked me very casually – it was very obvious what the right answer was.

Were you shocked?

It was outrageous, but that was the 80s. From my point of view, it was none of his business, but if I'd gone in with an engagement ring on my finger I wouldn't have got the job.

When you did have a child in your 30s, you went back to work after two months. Was there pressure to return so soon?

That was my misguided decision. I worked up until the day that Oscar was born; I had a conference call that morning. It was a busy time of year and I went back very early. But I'd stress that I was not told that I had to, and for me, personally, that was a mistake. I wasn't ready, physically.

You suffered from postnatal depression and ended up on a psychiatric ward for nine weeks. When did you know you were seriously ill?

I didn't see it coming. The first few weeks after Oscar was born were blissful. Depression is really odd; it doesn't always hit straight away. I can vividly remember coming home and feeling very well, but I wasn't sleeping – and sleep deprivation, I'd discover later, is huge. But I don't think I knew that postnatal depression existed.

How did it translate to quitting your job and deciding to become a writer?

There's an enormous amount of guilt attached to postnatal depression; you feel that you've failed everyone around you, particularly your child. But one of the doctors said: "You've got to your 30s without this happening, you did lots of other challenging things, so don't start changing your life story." There are a lot of people in the medical profession who'd say: "Don't take on new challenges." That would have been the worst thing for me.

Is it true you owned a greyhound that won the Irish Derby?

Ah yes, Tain Mor. It was 1976. My father originally bought the mother of the dog and we started to breed – and we still do. I went to an all-girls' convent school, so you can imagine the nuns were unimpressed that I was spending time down the dog track.

Is there any connection between greyhound racing, investment banking and becoming an author?

There's an internal logic. Having spent many years working very hard, it does give you stamina. To write books, you have to be slightly obsessive, because nobody needs you to write a novel, and it's going to take a lot of hours to make it a book that you want to read.