It's tough being a teacher in Ireland today, as this week's conferences show clearly. A pay freeze for 2009, an average pension levy of 7.5pc, an appointment embargo on posts of responsibility, increased pupil-teacher ratios and the loss of up to 1,000 primary teaching posts can only dampen moods in already demoralised staffrooms.

Yet, despite these current hardships, spare a thought for Irish teachers abroad whose daily working conditions would cause all the teaching unions in Ireland to strike permanently.

One such teacher is Sr Mary Killen, a 'tough-as-nails' Irish Sister of Mercy from Phibsboro, Dublin, who has spent the past 30 years teaching and managing schools in Kenya.

When she first came to Nairobi in January 1976, her honeymoon period was short-lived.

She arrived in the city on Thursday, on Friday she was interviewed and by Monday morning she was acting school principal.

In Kenya this is a dawn-to-dusk duty, for primary schools open their gates at 6.30am to allow preparation time before the classes officially start at 8.15am.

Although officially classes end between 3pm and 4pm, supervised study, tuition, games and extra-curricular activities often mean pupils don't leave until after six in the evening.

Sr Mary spent 14 years in Our Lady of Mercy, a middle-class primary school, until disaster struck in 1985 when the Kenyan government introduced new regulations forcing parents to pay for an extra year of primary school. The result was swarms of slum children descending on the capital's streets with nowhere to go and nothing to do.

In an attempt to get 100 of these children off the streets and into her classrooms, Sr Mary introduced a radical integration policy in the school that greatly angered the middle-class parents.

"The mix didn't work well," she sighs.

Firstly, the slum children ran an alternative 'sex education' programme with a practical rather than theoretical focus. They introduced gambling to the school; using the tiled floor of the toilets as a board. Having sharpened their skills on the streets, they cleaned up all round them.

Some older children set up racketeering scams, forcing classmates to pay money to prevent being reported to the class teacher. Lastly, they introduced scabies, tapeworm and ringworm to the pupil population, much to the horror of the middle-class parents.

"Once ringworm gets into your house it is very difficult to get out. The children bring it home, then the parents get it, even the towels have it," she says.

To compound these problems, the teachers were inclined to blame the slum children for everything that went wrong. Whether it was missing footballs, ties or school stationery it was always the slum children that became the prime suspects.

At that time children brought their lunches to school. This served to highlight even more starkly the existing class divisions for the slum children had no lunch boxes and many went hungry. But they refused to join the queue for free food that the school had provided especially for them.

"Even if they were starving they wanted to show the other children they didn't need it," Sr Mary remembers.

There were other challenges besides. School uniforms were liable to be stolen in the Mukuru slum and, for many, arriving with polished black shoes and clean white socks was a step too far. In the rainy season school books were sometimes lost when children dropped them while wading through a swollen river to get to school. If the books didn't get wet they could be burnt when mothers used pages as kindling for the evening fire.

With an estimated 400,000 residents, the cardboard-and-tin slum of Mukuru is the second largest slum in Nairobi, but one of its poorest. Like all shantytowns, it is densely populated with inadequate sanitation and water services. It is prone to fires and flooding -- 30 people drowned last year when a flash-flood caused by heavy rains swept their shacks away.

In 1985, in response to these needs, the Irish Sisters of Mercy set up the Mukuru Promotion Centre. Today, assisted by Irish Christian Brothers and Marianist priests, over 6,000 slum children now receive an education and 60 street boys are taught a trade.

Although heart-warming to hear the success stories of ex-street children who received their education and a chance in life thanks to the Sisters of Mercy and their donors in Ireland, the smile quickly fades when you hear the dangers faced by a child living on the streets.

Few people in Nairobi know more than Sr Mary just how deadly this life can be.

"In my 30 years of work with street girls," she says, "I've only met one girl over seven years of age who hasn't been sexually abused."

Sr Mary recounts the story of a gang of 10 "very wild street girls" between 10 and 12 years of age that would make St Trinian's schoolgirls look like angels.

The girls approached her in 1991 requesting the opportunity to begin primary school. None had attended school up until then.

"They were like wild cats, spitting and scratching," she recalls. One day, one of them took a knife to her housemother's throat after being corrected for inappropriate behaviour.

Something strange was causing the girls to lash out uncontrollably. It was only months later during art therapy that the full truth of their street experiences was revealed.

When asked to draw what made them really angry, the girls drew savage dogs with gaping teeth and drooling mouths. Yet, strangely, none of them had ever been attacked by dogs.

They then told how one day they met some Europeans who offered them drugs and filmed them carrying out acts that were so depraved it took them months before they could even speak about the horrors of that night.

All the girls had worked as prostitutes before coming to school and their persistent pimps would follow them from prison to school, attempting to lure them back onto the streets again.

"There was no way of stopping them if they wanted to go back," says Sr Mary. "Some of the girls couldn't live without the money and the drugs."

Out of the group of 10, two went back with their pimps, five finished secondary school and three have since died of AIDS, an illness they probably contracted while on the streets.

Although described as "a humble woman of God", Killeen is no shrinking Sr Violet. She is a fighter, hardened by her many battles fought with land-grabbers over school properties.

Undeterred by veiled threats on her life, she even organised a demonstration of parents and students in a country where it is illegal to protest and demonstrators are routinely arrested and beaten by the police.

When not setting up schools and rehabilitation centres for street children, Sr Mary enjoys walking and reading. A voracious reader, on her bedside locker lie The Portrait of an Artist as a Young Girl edited by John Quinn and This Opened my Eyes by Fr Varkey SJ.

When I asked her what country she thought of as home, Sr Mary answered by saying that she has now lived more of her life in Kenya than in the country of her birth.

"I have no idea what it would be like to live and work in Ireland. I am more familiar with life in Kenya," she said before adding, "still, I love to get back home and to see family and friends."

This usually happens once every two years. Home in Nairobi is a community of eight sisters: four from Kenya and four from Ireland. Their convent-come-house was formerly built as a doctor's residence on the leafy grounds of Nairobi's exclusive Mater Hospital.

On hopes for the future, Sr Killeen dreams of a day when every child will have access to a basic education. In the meantime, this relentless Sister of Mercy continues her fight against perverts, pimps and penny-pinching politicians, a religious habit she will never get out of as long as street children continue to roam the streets of Nairobi city.