María O'Neill, a writer, began her career in theosophy, and later became interested in spiritism, to which she devoted much of her life until the end of her days. He participated in the 1st Portuguese Spiritist Congress and gave FEP the most dedicated collaboration. He was part of the editorial staff of the magazine Espiritismo, in which he always collaborated.

Person «open to all the anxieties and worries of thought, his life was an apostolate of generous and beautiful ideas». Her literary work, quite extensive, included a collection of children’s stories, considered the best in Portuguese at that time, and which were translated into several languages. It began literarily with a book of poetry, «Nimbos», prefaced by Bulhão Pato, to which "followed other works, some of remarkable value, such as «The Struggle of Feelings», «Illusion Undone», «A Drama of Jealousy», «Love is suffering», «A happy engagement», «Love Psalms», «Real Life», «Female Souls», «The Viper», «Loose Leaves», «The Collar of Vera», «Love everything gets», among others.

Their philanthropic activity was no less fruitful, because they were made to be discreet by their moral greatness, to do good without concern, to distribute the savings and the performance of their literary works with motherly simplicity to those in need.

The companion and teacher, who was dedicated to the Portuguese Spiritist Federation like few others, had the ideal that she made visible through the demystification and purity she knew how to serve... Woman of great culture, member of the Academy of Literature, had always been a brilliant spirit and superior character... Within the FEP, it was a coordinating and harmonizing element between those who were sometimes in disagreement, which led them to reconsider their attitudes, which were sometimes considered as insoluble.

Maria O'Neill surprised all who heard her with her easy words, clarity and clear reasoning. A true woman of arms is developing in conferences, giving special support to the Spiritists from the Algarve and Alentejo who often invited her to lectures. Several praising references to Maria O'Neill could be found in "Comércio do Porto" (17-11-1929), "Gazeta de Coimbra" (28-11-1929), "O Primeiro de Janeiro" (28-11-1929), among many other famous newspapers of the time.

In 1930 she went to Brazil, where she gave a series of lectures on literary and spiritist topics. The success is great in the countries of Santa Cruz. She returned to Lisbon and went back to Brazil, this time in February 1932, where she was expected to attend many conferences on spiritism. Her health was quite serious when she landed in Bahia and decided to return urgently to Lisbon.

She died on the open sea, two days after she had begun her return journey, on 23 March 1932. Her body was thrown into the sea, as preferred by the Hindu mystics with whom Maria O'Neill had a deep connection, as a student of Annie Besant and Blawatsky, while their immortal spirit, in search of new horizons, demanded further stations of spirituality.