In the 1800s, many people in Ireland were poor. They didn't have enough money to eat, pay their bills and buy clothes. They also didn't have a lot of education. Some people didn't even have a home to live in.
Catherine wanted to help the poor people in Dublin. She went to the streets and saw how hard it was for them. She decided to give them a place to live and food to eat. She also helped some of them learn to read and write. She also helped them get jobs.
She prayed everyday and loved God very much. She knew that God needed her to help the poor people in Ireland. So she started the House of Mercy in Baggot Street in Dublin.
The House of Mercy was a special place for the poor people in Dublin to come and stay. The rich people who lived in Baggot Street noticed that the house was full of poor people and they started to give her money so that she could keep the House of Mercy running.
After a few years, the money that she had left started to run out. So she had to depend on other people to help her. Then she realized that she was going to need a religious order so that she could have more help.
So she asked her friends to help her and they all became Sisters of Mercy. They became "Walking Nuns" and they would go to different places to help the poor people.