Mimi Irwin
It appears that Mimi may have been adopted by the Thomas family. She was born in Iowa and her name is shown as "Minnie J. Thomas" in the 1870 Utah census with the David Thomas family.
The Thomas Family emigrated from England in 1857. They sailed on the George Washington, arriving in Boston on April 20. The family appears in the 1860 census living in Iowa City. They came to Utah in 1868. David appears as the head of the family in the memorandum book. His daughters, (with married names) are also included in his family group. Mimi Irwin (6) also appears. They all appear living together in Salt Lake City in the 1870 census.