I came from a background in which education was extremely important. For starters, my mum was an English and French teacher who drummed the importance of learning into us on a daily basis. Plus, of course, I grew up in Ireland where education is a powerful motivating force. Perhaps, historically, the Irish have always seen it as a way up and out.

My mum taught, with amazing results, at the local state school. Her pupils were often underprivileged kids who came from families of 12 and more. But my two sisters and I went to Loreto Beaufort, a private convent school in Rathfarnham, near Dublin. Maybe my mum thought she would compromise us if we went to her school; maybe there was a little bit of middle-class snobbery in it.

Either way, Loreto Beaufort was a pretty academic place. There was a sense that everyone there was probably being trained as a future lawyer or doctor. Acting certainly wasn’t on the list of desirable professions. It was seen as something a bit loose and depraved, so when I started acting professionally at 15, I think the view of most teachers was that I had somehow “gone to the dark side”.

Some, but not all. I did have a particular rapport with Sister Mary Rose O’Kane, who was always incredibly encouraging and interested in the double life that I was living outside school. Surprisingly perhaps, she didn’t teach drama or even English. In fact, she taught maths.

Now, I was not a stupid child. I was good at foreign languages. I was good at English. I was even good at science. But maths completely foxed me. I could learn and recite the telephone directory if need be, but Pythagoras was lost on me, and this despite the fact that both my elder sisters were brilliant mathematicians.

Sister Mary Rose seemed to understand how painful comparisons with my siblings must be and she constantly resisted the temptation to ask: “What the hell happened to you when they were dishing out the maths gene?” Plus, at a time when maths was a key subject and people thought: “What’s the good of someone who can’t add up?” Sister Mary Rose didn’t make those judgments at all. So I was lousy at logarithms. Big deal. She treated me with huge respect and understanding that my strengths lay elsewhere.

There are certain teachers, I suppose, who just “get” you and Sister Mary Rose “got” me. Although I hide it well, like a lot of actors, I’m probably barking mad. And maybe she related to that rather eccentric part of me because she herself was not what you would call a conventional nun. She was quirky and dynamic and she oozed this anti-authoritarian streak. I used to wonder how she had ended up as a nun, whether she’d had a choice in the matter, because at that time in Ireland a lot of girls just found themselves sent to convents at the age of 16.

Don’t get me wrong, I’m not saying she wasn’t devoted to God. She was a deeply religious woman. But at the same time, I had this feeling there was something very much bigger going on inside Sister Mary Rose and that she was making the best of the situation she was in.

Sadly, I lost touch with her after I left Loreto Beaufort. Actually, I was asked to go before taking the Leaving Certs (the Irish equivalent to A levels). By that time, the acting had become such a big part of my life that I was considered beyond redemption, I think. I was deemed to be “an unsuitable influence on the girls”.

I could be bitter, but actually I still have plenty of positive things to say about my education. Thanks to my mum’s guidance and help I still did well in those exams and I think that the high standard of the Catholic education I received equipped me for life in many ways. Now, I’m even trying to get my own kids into the Catholic school around the corner.

With hindsight, you tend to focus on the good things and the good people. Sister Mary Rose was definitely one of them.

Dublin-born actress Dervla Kirwan, 35, has appeared in TV dramas including Ballykissangel, 55 Degrees North and True Dare Kiss. She lives with her partner, Spooks star Rupert Penry-Jones, and their two children. She was talking to Daphne Lockyer.