Counting the ancestors


Madam, – Prof Noel Mulcahy (December 8th) asks if Ryan Tubridy has a unique claim to descent from Brian Boru (further to my article in the “Ancestors” supplement, December 1st).

The answer is No. The emerging consensus in genealogy is that the professor, and almost everyone in Ireland today, probably descends from Brian Boru. And it is not special to Brian Boru. Most Irish people today probably descend from most Irish people alive in 1000 AD. That is, we descend from everyone alive in Ireland back then apart from the minority who did not reproduce or whose line died out. We pick Brian Boru as an example because we know he is not in that minority. We know his line did not die out because we can prove descents from him to people alive today.

In this regard, Ryan Tubridy does have something the professor and I do not have – he can prove every step of the way back to Brian Boru. This is not unique – millions of people in the West can prove this, but far from everyone in the West can do so. Ironically, few people in Ireland can prove this.

Prof Mulcahy also notes the exponential growth in the number of ancestors one has, doubling with each generation as you move back. He estimates the number of ancestors you have in 1000 AD as about two to the power of 30, which is reasonable. But he then incorrectly says this is 1.1 million. In fact it is 1.1 billion, more than the population of the entire world at the time, never mind just Ireland. And go back just seven more generations and you need more than 100 billion ancestors – more than the number of humans that have ever existed, past or present.

Clearly these ancestors cannot all be unique. The same ancestor must double up many times – perhaps millions of times. The cause of this is not (mainly) that people married twice. The cause is (mainly) that cousins married. Obviously every human marriage is a cousin marriage to some degree, but these numbers show that we are descended, even just in the past 1000 years, from thousands of cousin marriages where the cousins are closer than you might think.

If you consider (see again “Ancestors” supplement) that each of us has perhaps one million 10th cousins, we see that practically anyone we date will be related to us within remarkably recent historical times, much more recent than Brian Boru. We are all kissing our 15th cousin or closer without even knowing it. – Yours, etc,


School of Computing,

Dublin City University,


Dublin 9.