The Irish surname Hennessy is well-known around the world and not just in English-speaking countries. This isn't because it was a common or particularly well-distributed name in Gaelic Ireland, but rather because one man and his descendents established an internationally renowned eponymous brand. The picture below tells part of the story; there is more further down the page.
The original Irish form of the name is O hAonghusa, meaning descendent of Aongus (Angus), a Celtic god. It's original meaning isn't entirely clear. I've seen suggestions it means everything from 'choice' to 'strength' and 'energy' to 'love', so whatever the precise early intention, it's certainly got a vigorous masculine feel about it.
This is probably why it became quite a popular first name and, in the Gaelic tradition, was handed down to the descendents of the family by attaching the O.
In Gaelic tradition, personal names were used to denote familial heritage with the adoption of the O prefix - meaning 'descendent of' - followed by the ancestor's personal name. (The mac prefix is more specific, meaning 'grandson of', plus the name.)
In due course, these became what we know as surnames. Although most were subsequently anglicised, many prefixes were retained. (See Irish last names for more about the O and mac.)
The anglicised Hennessy was one of those where the prefix had fallen out of fashion by the mid-1800s. In fact, I can find no instance of an O'Hennessy in either the 1901/1911 censuses or the online civil registration records of births, marriages and deaths from 1864.
More recent records, which can't be examined for data protection reasons, may show some resurrection of the prefix, of course.
Jumping back to Gaelic Ireland, however, the O'Hennessy septs (family groups) were found in County Cork in what we'd now recognise as the coastal Rosscarberry-Skibbereen area, and the Fermoy-Mitchelstown area.
Others were in the north of County Offaly (King's) and moved eastwards towards Counties Tipperary and Clare.
Although the name was gradually carried further, a survey called Griffith's Valuation, which was carried out across the island between 1847 and 1864, clearly showed that the 1,281 Hennessy, Henessy, Henesy, Hennessey, Hennesy and Hensey households had not wandered too far. As you can see from the map, above, while a few outliers had headed north from familiar territories, the majority were found in the southern half of the island.
The largest groups were (in descending order) in Cork, Tipperary, Limerick, Kilkenny, Kerry, Clare and Laois.
Half a century later, these were still the main areas where the family name appeared. In 1911 the census recorded 4,224 individuals with the Hennessy surname, 338 Hennesseys, 27 Henessys and just 4 Henesys.
Just as the big story of Ireland is emigration, it follows that a number of Hennessys departed from the island in search of hope, work and fulfilment.
But because the name, no matter how it is spelt, has never been a 'BIG' name in its homeland, so the numbers immigrating into other countries has ever been huge
If you ignore the 'Cognac Effect' (see The Irish story behind the Hennessy cognac brand above), would the Hennessy surname have registered much impact around the world, at all? Probably not.
In relation to its original distribution and numbers in Ireland, the surname follows a fairly typical pattern of emigration.
In England, Scotland and Wales, the earliest census shows only 115 individuals with the name in 1841. The oldest of those born in Ireland was Margaret Hennessy, a widow living in Eltham, Kent, who gave her date of birth as 1771.
The numbers stepped up, decade by decade, (263; 327; 491; 590; 607; 804) until 1911, when there were still fewer than 1,000 Hennessys across the three nations. You can see the ratio of spellings in the graph above.
In the USA in 1850, there were 1,586 Hennessys of all spellings residing in the country, with just over half - 813 - having been born in Ireland. Many of these would have arrived in response to the Great Famine of 1847-1852. By 1900, the total figure was 11,379, and nearly 20% had been born in Ireland.
The 1940 census shows a striking change. While there was only a tiny increase in the overall number of 12,210 Hennessy individuals, only 577 (less than 5%) had started life on the other side of the Atlantic.
As you can see from the feature box, left, the Irish surname Hennessy is still growing, with the 2010 census recording just over 19,000 individuals.
Heading further afield, the occurrence of the family name Hennessy in Australia is tiny, but at least each of the states has a few representatives! The map below shows a likely underestimate ofdistribution in 2014.