First found in county Kilkenny, where they were seated from very ancient times, long before the Norman Conquest in 1066.

Ireland became inhospitable for many native Irish Families in the 19th centuries. Poverty, lack of opportunities, high rents, and discrimination forced thousands to leave the island for North America. The largest exodus of Irish settlers occurred with the Great Potato Famine of the late 1840s. For these immigrants the journey to British North America and the United States was long and dangerous and many did not live to see the shores of those new lands. Those who did make it were essential to the development of what would become two of the wealthiest and most powerful nations of the world. These Irish immigrants were not only important for peopling the new settlements and cities, they also provided the manpower needed for the many industrial and agricultural projects so essential to these growing nations. Immigration and passenger lists have documented the arrival of various people bearing the name Brennan to North America: James Brennan, a bonded passenger, who arrived in Potomac in 1731;Timothy Branen who settled in Placentia, Newfoundland, in 1744; Laurence Brennan, on record as a laborer in St. John's Newfoundland in 1779.

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