Short answers, pulled from the story.
Buddhism was introduced to Sri Lanka in the 3rd century BCE, during the reign of King Devanampiya Tissa (307-267 BCE). According to the Mahavamsa and Dipavamsa chronicles, it was brought by the elder Mahinda and the nun Sanghamitta, both said to be children of the emperor Ashoka.
As of the 2012 census, 70.1% of Sri Lanka's population practiced Buddhism. Theravada Buddhism is the largest and official religion of the country under Article 9 of the Sri Lankan Constitution.
Henry Steel Olcott was an American who arrived in Sri Lanka in 1880 alongside Madame Blavatsky of the Theosophical Society. He converted to Buddhism, co-founded the Buddhist Theosophical Society, sponsored colleges including Ananda College in Colombo, published a Buddhist Catechism in 1881, and helped grow Buddhist schools from three at the time of his arrival to 429 by 1940. He also sat on the committee that designed the Buddhist flag.
There are two main Nikayas currently in Sri Lanka. The Siam Nikaya was founded in the 18th century with the help of Thai monks and restricts higher ordination to the Radala and Govigama castes. The Amarapura-Ramanna Nikaya was formed in 2019 through a merger of the Amarapura Nikaya (founded in 1800 via Burmese monastics) and the Ramanna Nikaya (founded in 1864); it is the largest Nikaya and allows ordination regardless of caste.
The Panadura debate of 1873 was a public religious debate between Buddhist monks and Protestant missionaries, and was widely seen as a victory for the Buddhist monk Migettuwatte Gunananda Thera. It was one of five major public debates held between 1865 and 1873 covering topics such as God, the Soul, karma, rebirth, and nirvana. The debate's fame spread internationally and directly inspired Henry Steel Olcott to travel to Sri Lanka in 1880.
The original order of nuns established by Sanghamitta died out in Sri Lanka in the 11th century. In 1996, eleven Sri Lankan women received full bhikkhuni ordination at Sarnath, India, through the efforts of the International Buddhist Women's Association Sakyadhita. The higher Theravada leadership in Sri Lanka has not recognized these ordinations as valid, and women who wish to live a monastic life may do so as Ten Precepts Angarika nuns.