Thomas O'Neill was born in Killear (now spelled Killare), Co. Westmeath, Ireland. He married Margaret Byrne and had 7 children. Thomas and Margaret emigrated to Quebec in 1831, but left their 3 children (Ann b. 1827, Hugh and Catherine, b. 1829) behind in Ireland. Ann and Hugh were left with Thomas's parents, while Catherine was left with Margaret Byrne's sister Eliza Byrne who married Thomas's brother John. In Montreal, Thomas and Margaret had 3 more children. John in 1835, Elizabeth in 1837 and Margaret in 1841.
In 1841, the older children finally went to Quebec and met their Canadian born brother and sisters. But their mother Margaret had died after the birth of her daughter Margaret. Thomas later married his 2nd wife, Rose Warnock, and they had 3 children, Elizabeth in 1858, Thomas in 1864 and Patrick James in 1865.
Thomas lived at 22 St. Claude St. in Montreal from 1852 to 1858 according to the City Directory. He was the Sheriff's Officer according to the Directory.
From the book "A Worthy Daughter of the O'Neills" we read: "When Mr. and Mrs. O’Neill [Thomas and wife Margaret Burns] sailed to America in 1831 they left their children behind. Ann and Hugh stayed with Mr. O’Neill’s parents; Catherine, only nine months old, was taken by her mother’s sister, who had married Mr. O’Neill’s brother. The parents thought it best to endure the pain of separation from their little ones until a suitable home could be madefor them in the land of promise, beyond the sea, whither they were turning their faces. It took Mr. O’Neill six years to accomplish this design. His wife, who belonged to an aristocratic family, was disinherited for marrying the man of her choice. To earn for her what she had given up for him became his ambition. To its attainment the resolute young Irishman brought a good education, a generous nature, and rare intellectual gifts. Thomas O’Neill had a meager share of this world’s goods when he reached Quebec, but a great fund of confidence in God, in his own youth, and ultimate success, for a beggar meeting him on the street solicited alms. Mr. O’Neill unhesitantly handed the mendicant his entire capital—twenty-five cents. He managed, however, to push on to Montreal. … He did heavy labor … he tried harvesting … broke stones … until his hands were disabled. He got medical aid from Dr. Nelson, the Patriot of ’37. [This was an uprising of the French-speaking “lower Canada” before it joined the rest of the region; it was short lived.] The doctor obtained [for Thomas] the position of sheriff in Montreal and they started a lifelong friendship. In short time Mr. O’Neill began a wood business. He prospered beyond his own hopes. His earnings were invested in real estate in Montreal, then rapidly increasing in value. At last his wife could enjoy life such as had been hers in her father’s home; at last he could bring his little ones to the cheer and plenty to which he had looked forward. But God’s ways are not ours. In 1841 Ann, Hugh and Catherine set out from Ireland with their uncle, Mr. O’Neill’s brother. When the party reached Montreal, Thomas O’Neill was at the dock to meet them. But where was Mother, the children asked. In answer the father sobbed: “Two months ago your mother went to heaven.” Mrs. O’Neill’s death was a great sorrow to the family; to Catherine’s sensitive, affectionate nature it was a crushing blow. She had dreamed of her mother during the years of lonely separation—she was destined to feel the pain of her loss throughout her entire life. The bereaved children gathered more closely around their father, clinging to him in mute affection as they made their way home. There they saw their little sister Margaret, only two months old. There they were greeted again by Elizabeth, Mary and John, whose acquaintance they had made at the wharf; the Irish half of the family was so perfectly cemented that day to the Montreal half that all the years which rolled over them since then have only made the jointure more solid, more durable, more happily visible. Baby Margaret did not long survive, however, a few short months completed her existence, but the others grew to manhood and womanhood, shedding as they traveled life’s way kindly blessings on all who were fortunate enough to come in contact with them. Ann, the eldest took control of the household. Catherine, Elizabeth and Mary were sent to boarding school at Pointe aux Tremble, where they remained four years with the Congregation of Notre Dame (1841–45). When Mr. O’Neill heard of Mother Mary Rose Durocher’s boarding school at Longueuil, with the newly formed Sisters of the Holy Names of Jesus and Mary, he decided to place his daughters there The two girls were there from 1845 until their entry to the convent in 1847, taking their vows in the following years."