Kathleen was born in the townland of Mullafarry, near Killala in County Mayo. In 1882, the family moved to Shrule in County Longford where her father took over as the clergyman of the Ballymahon parish. Later on in her young life, in 1886 Kathleen and her family moved to Cong, a village bordering Mayo and Galway to where her father's parish was being funded by the Lady Ardilaun of Ashford Castle. Kathleen was sent to the Alexandra College, of which Lady Ardilaun was the patron, in Dublin which she attended till she was sixteen years old. She was distantly related to Countess Markievicz through her aunt's marriage. Growing up in the aftermath of The Great Famine (1845 – 1852), Kathleen was deeply saddened by the deadly diseases and poverty suffered by the people in her local area. This led to her desire at sixteen, when she left school, to become a doctor.