2000
#642
National surname rank
Recorded bearers 48,296
First available Census row
Per 100,000 17.90
Uncommon Last name
Brennan
A surname of Irish origin meaning "descendant of Braonán," derived from an Old Irish word for "rain" or "moisture."
According to the 2020 US Census Bureau surname tables, roughly 54,820 Americans carry the last name Brennan. That puts it at #697 in the national surname ranking, appearing at a frequency of 15.99 per 100,000 people (about 1 in 6,252 residents).
This page is the full Name Census profile for the Brennan surname. You will find the Census Bureau frequency data, a multi-census history view, an ancestry and ethnicity breakdown based on self-reported demographics, the name's meaning and origin where available, and answers to the most common questions people ask about this surname.
For British records, Name Census UK has a British surname profile for Brennan with 1881 census detail, origin facts and modern UK distribution where available.
Bearers in the US
55K
1 in 6,252
Census rank
#697
2020 decennial data
Per 100,000
16.0
Frequency rate
Recorded bearers
48K
uncommon in the US
Popularity narrative
The Census Bureau recorded 47,806 bearers of the surname Brennan in its 2020 decennial surname file. At a rate of 15.99 per 100,000 residents, it holds the 697th position in the national surname ranking.
Among Census respondents with the surname Brennan, the largest self-reported group is White at 91.7%. The next largest groups are Hispanic (3.4%) and Two or More Races (2.9%).
Origin
Meaning and origin of Brennan
The surname Brennan originated in Ireland, derived from the Gaelic name O Brannagain, which means "descendant of Brannagan". Brannagan was an ancient Irish personal name that meant "raven" or "blackbird". The name was first found in County Kilkenny and later spread to other parts of Ireland.
The earliest recorded instances of the surname date back to the 13th century. In medieval Irish records, it was often spelled variations such as O'Brenaghan, O'Brennan, and O'Branagan. The Brennan name is believed to have been present in Ireland since before the arrival of the Normans in the 12th century.
One of the earliest recorded Brennans was Dermot O'Brennan, a chief of the Uí Duach sept in County Kilkenny, who lived in the late 12th century. In the 14th century, the Annals of the Four Masters mention a notable Brennan named Donnchadh O'Brennan, who was the Chief Poet of Ireland.
During the 16th and 17th centuries, the Brennan name became more widespread across Ireland. In 1659, the Petty's Census of Ireland listed several Brennan families living in counties such as Kilkenny, Waterford, and Cork.
A notable Brennan from the 18th century was Sir Jonah Brennan (1736-1801), an Irish physician and anatomist who served as the President of the Royal College of Surgeons in Ireland. In the 19th century, Michael Brennan (1827-1904) was a prominent Irish-American labor leader and politician in New York City.
Other historical figures with the Brennan surname include Theresa Brennan (1856-1939), an Irish-American philanthropist and founder of the Sisters of the Holy Child Jesus; and Louis Brennan (1852-1932), an Irish inventor and engineer who developed the first guided missile system.
Demographics
Ancestry and ethnicity for Brennan
Among Census respondents with the surname Brennan, the largest self-reported group is White at 91.7%. The next largest groups are Hispanic (3.4%) and Two or More Races (2.9%).
The bar chart below shows how Brennan bearers described their own race and ethnicity on the 2020 Census form. The Census Bureau groups responses into six broad categories: White, Black or African American, Hispanic or Latino, Asian and Pacific Islander, American Indian and Alaska Native, and Two or More Races. When a category has too few respondents for a given surname, the Bureau suppresses the figure to protect individual privacy, which is why some names show fewer than six slices.
Percentages are shown for every Census year so the breakdown stays comparable over time. When the source file also includes raw headcounts, Name Census shows those alongside the percentages in the legend.
Keep in mind that these are self-reported numbers. A person's surname does not determine their race or ethnicity, and the distribution you see here reflects the specific population who happened to carry the Brennan surname at the time of the 2020 Census, not any inherent property of the name itself.
- White91.7% · 43,833
- Hispanic or Latino3.4% · 1,631
- Two or more races2.9% · 1,390
- Asian and Pacific Islander0.9% · 430
- Black or African American0.9% · 421
- American Indian and Alaska Native0.2% · 101
Timeline
Historical Census data for Brennan
Brennan appears in 3 published Census surname files: 2000, 2010, 2020. The cards below show how the name's rank and bearer count changed across each release.
2010
#701
National surname rank
Recorded bearers 49,238
+942 bearers (+2.0%)
Per 100,000 16.69
Rank movement Down 59 places
2020
#697
National surname rank
Recorded bearers 47,806
-1,432 bearers (-2.9%)
Per 100,000 15.99
Rank movement Up 4 places
| Year | Rank | Count | Per 100K | Count change | Rank change |
|---|---|---|---|---|---|
| 2000 | #642 | 48,296 | 17.90 | First available Census row | First available Census row |
| 2010 | #701 | 49,238 | 16.69 | +942 bearers (+2.0%) | Down 59 places |
| 2020 | #697 | 47,806 | 15.99 | -1,432 bearers (-2.9%) | Up 4 places |
For 2020, the Census Bureau published race and Hispanic-origin columns as counts rather than percentages. Name Census converts those counts back into shares so the ancestry section stays comparable with the older surname files.
Year on year
2010 vs 2020 Census
How has the Brennan surname changed between Census years? The chart shows bearer count side by side, and the table compares rank, count, and frequency.
Census year comparison
20102020
| Metric | 2010 | 2020 | Change |
|---|---|---|---|
| Rank | #701 | #697 | 0.6% |
| Count | 49,238 | 47,806 | -2.9% |
| Per 100K | 16.69 | 15.99 | -4.2% |
Between the 2010 and 2020 Census, the number of Brennan bearers went from 49,238 to 47,806 (-2.9% change). The surname moved up 4 positions in the national ranking, going from #701 to #697.
Notable bearers
Famous people with the surname Brennan
FAQ
Brennan surname: questions and answers
How many people in the U.S. have the surname Brennan?
Name Census estimates that about 54,820 living Americans carry the surname Brennan. Using the current population baseline, that works out to roughly 1 in 6,252 residents.
How common is Brennan?
Brennan ranks #697 in the 2020 Census surname tables and is classified on this site as "Uncommon." The Census recorded the name at a frequency of 15.99 per 100,000 residents, which is about 16 people out of every 100,000.
How many people with this surname were counted in the Census?
The raw 2020 Census file counted 47,806 people with the surname Brennan. That is different from the site's living-bearer estimate (54,820), which projects the surname's present-day count by applying the Census frequency rate to the current U.S. population.
What does 15.99 per 100,000 actually mean?
It is the Census Bureau's normalized frequency measure. A rate of 15.99 per 100,000 means that if you picked a random group of 100,000 U.S. residents, you would expect about 16 of them to have the surname Brennan.
Has Brennan become more or less common over time?
Between 2010 and 2020, the surname Brennan went from 49,238 recorded bearers to 47,806. That is a decrease of 1,432 (-2.9%). In the national ranking it rose from #701 to #697.
What does the Census say about the background of Brennan?
Among Census respondents with the surname Brennan, the largest self-reported group is White at 91.7%. The next largest groups are Hispanic (3.4%) and Two or More Races (2.9%). These figures come from the 2020 Census Bureau surname tables, based on how respondents described their own race and ethnicity.
Which group reports this surname most often?
White is the largest self-reported group for the surname Brennan in the 2020 Census, accounting for 91.7% (43,833 people in the source table).
What is the full ancestry breakdown?
Brennan appears across multiple self-reported groups in the Census data. The largest shares in the 2020 file are White (91.7%), Hispanic (3.4%), Two or More Races (2.9%). For 2020, the source file also published raw headcounts for each group, which is why this page can show both percentages and counts in the ancestry section.
Is this page using the latest Census data?
Yes. This page is using the latest surname file currently loaded on Name Census, which is 2020. The historical section above also keeps any older Census surname entries we have for Brennan (2000, 2010, 2020).
Does the Census include every surname?
No. The Census Bureau only publishes surnames that appeared at least 100 times in a given decennial Census. That means very rare surnames are excluded entirely, and a surname can appear in one Census release but disappear from a later one if it falls below the reporting threshold.
Why don't the ancestry percentages always add up to exactly 100%?
There are two main reasons: rounding and suppression. The Census Bureau rounds published values, and it may suppress very small cells to protect privacy. For 2020, the Bureau also published raw group counts rather than direct percentages, so Name Census converts those counts back into shares for comparability across census years.
What does Brennan mean?
A surname of Irish origin meaning "descendant of Braonán," derived from an Old Irish word for "rain" or "moisture." The fuller origin note on this page goes into more detail.
Where does the surname data come from?
All surname statistics on Name Census are drawn from the US Census Bureau's decennial surname frequency tables. These files list every surname that appeared 100 or more times in the 2020 Census, along with a count, a per-100,000 rate, and a self-reported demographic breakdown. You can read the full explanation on our methodology page.
How does Name Census estimate living bearers?
For surnames, Name Census does not age cohorts the way it does for first names. Instead, it takes the Census Bureau's published frequency for Brennan (15.99 per 100,000) and applies that rate to the current U.S. resident population to estimate how many living Americans have the surname today.
How many Americans have the surname Brennan?
For a faster, more casual read, check HowManyOfMe.org — our sister site built around that single question.