It was September on a windswept beach in County Mayo. We had a bright day. It had been quite cold before that. We’d parked the car and brought a picnic with us. I can remember the glorious scenery, this huge wide-open beach. My sister is a great photographer and so I said to her when we’d eaten the picnic, “Why don’t you go off and take some photographs? I’ll look after Mik.”

I had my back to her baby daughter, Mikaella, my niece, and I was chatting away to her as Marie went off.

Marie, my older sister, and Mikaella, 18 months, were visiting me in Dublin and we had decided to rent a cottage by the sea.

It was an idyllic time. I’m one of seven children and we’re a close family. I was already an aunt to my brother’s boys by this time, but Mikaella was special – the first niece. (I don’t have any children of my own.) As Marie headed off to take photographs, Mikaella was sitting amid the remains of the picnic stuff. She had been gurgling and then she went quiet. I didn’t know it in that second, but she had picked up a piece of orange left on one of the plates. She put it in her mouth.

And she started to choke.

In that moment, everything started to go slow and strange. As I turned round I saw that she had gone blue and I was thinking, why have you gone that colour? Why is everything so strange?

And then I was suddenly thinking, oh, my God. She’s choking. I picked her up, whacked her on the back. It wouldn’t come out. I was holding her and looking around for help. No one else was on the beach. I hit her on the back and it wouldn’t come out.

I finally hit her again and the piece of fruit came out. I started to shake and she started to cry. I have never in all my life been so glad to hear a child cry. She cried and cried. It was more from the shock of me hitting her than anything else, from slapping her back. But then it was almost like the sun came out from behind the clouds. Within a minute she was playing again. She was absolutely fine.

I wasn’t. I was shaking and in shock. My sister came back about 15 minutes later. By that point the picnic was packed away and we were drawing Mikaella’s name in the sand.

I couldn’t tell my sister what had happened. I was still too shocked and had already gone through it in my head a thousand times. What if I hadn’t turned around when I did?

It wasn’t until about five years later that I told Marie about the incident. She was just glad that nothing had happened. She realised it must have been horrible. And she completely got it. She has since had three other children and I am godmother to the next child she had. So she joked, “I wouldn’t have made you godmother to the next one if I’d known.”

My mum was there when I told Marie and we talked about it together. Marie sometimes gave her children a piece of fruit to take to bed at night and Mum then confessed – we’re a good, Catholic family – that she was always in fear of us choking as kids, and it really worried her if Marie gave her kids fruit to eat in bed at night. She said it always made her edgy.

So it was helpful that we all talked about this. I hadn’t realised until then the impact the choking had had on me. I haven’t talked about it with Mikaella even as an adult. She’s 20 now and studying at university in the US. But I think that if we did talk about it, it wouldn’t bother her.

In some ways I almost felt like I was protecting my sister by not telling her what happened. It was her first baby and I didn’t want her to feel that she had to be super-careful if someone else was looking after her child. It was a whole kaleidoscope of emotions. And it changed the way I felt about looking after children. What kind of an aunt was I if I look after her for five minutes and something like this happens?

It was the moment I realised how much everything depends on crossroads in life. If something happens, it all goes in one direction. And if an alternative happens, then … That’s what I kept thinking about afterwards. That moment needn’t even have happened. Or I could have turned around 15 seconds later and been too late. Or I could have not known what to do when a child is choking. I think I had seen my mum do something like that to one of my brothers and sisters.

But I just kept thinking, what if? It would have changed everything. It would not just have ended Mikaella’s life. The entire landscape of my family would have changed in that brief time. That stayed with me, the knife-edge that we’re always on in life.

For a while I avoided looking after children. It was strange because I had looked after loads of children from a young age, being from such a large family. Babysitting was my after-school job as a teenager. But after this incident, the next time I went to see my sister and her family in Berlin, I made sure that we were always together. If there was a scenario where I was in charge of the baby, I tried hard to make sure someone else was in the room.

I became overly watchful around children and still am. I have 17 nieces and nephews. The youngest is four and she and her seven-year-old brother have just been staying with me in Dublin. I’d like to think I’m a relaxed, fun aunt. But I’m so watchful. Much more so now than I would have been had this never happened. It’s always in the back of my mind.

The thought of the guilt is one of the things that frightens me most. And it is still my greatest fear. “If something terrible happened …” What if I had been responsible for the death of my sister’s child? None of us would have ever got over it. It’s almost unimaginable. The sense that everything would have changed if that had happened.

I have a sensitive radar for any story in the newspapers pertaining to this kind of thing. Stories of fathers and mothers reversing in a driveway and killing their toddler. Those things break my heart. I can never stop thinking about that moment when they realise what they’ve done and what has to happen next. How do you get over that?

Is there such a thing as just accidents? Accidents do just happen. In normal life – for want of a better word – people don’t deliberately go out to hurt a child. But still these things happen. And if something terrible happens as the result of an accident … It stirs up so much.

It’s hard for us to accept that we’re not invincible. If you’re looking after a child there are the obvious things that you think about, such as keeping them away from fire and knives. When the choking incident happened, we were staying in an old Irish cottage and I can remember thinking that I wouldn’t have dreamed of letting my niece anywhere near the fire, so how could this other thing have happened? It was the unexpectedness on that day of being in this big, wild, open space.

We were in this happy bubble where it felt as if nothing could touch us. It was like a tear in the perfect day.

• The House of Memories by Monica McInerney is published by Macmillan, £12.99