The claim: Former Prime Minister Gordon Brown says 500,000 Irish people live in the UK.
Reality Check verdict: 331,000 Irish people live in Britain, according to the most recent ONS statistics.
He said that illegal immigration was the "real problem" and it was important that EU migration figures were not taken "out of proportion and out of perspective".
The UK has a population of 63.7 million, of which 5.3 million (8%) are non-British, and just over half of those - 2.9 million (5%) - are from Europe.
But which EU countries have the most citizens living in the UK?
We looked into this by checking the latest Office for National Statistics (ONS) figures, collected between January and December 2014 in its Population of the United Kingdom by Country of Birth and Nationality report.
They tell us that Polish nationals represent the largest group of EU nationals living in the UK overall, with 853,000 living in the UK.
The Irish were the second largest group, with 331,000 residing here. Romanian and Portuguese nationals are in joint third place, with 175,000 people from each country living in Britain.
These figures include only those who live in the UK long term: what the ONS calls the "usual resident population".
To count, people have to be living here for 12 months or more.
Those visiting for less than a year - for example those studying in the UK on a short course - would not be included.
In its report, the ONS defined nationality as "that stated by the respondent during the interview".
Italy, Spain and Greece - countries affected by the Eurozone crisis and high levels of unemployment - had 170,000, 131,000 and 54,000 respectively.
Polish citizens are also the largest group of foreign nationals living in the UK overall.
Read more: The facts behind claims in the EU debate
On Monday, former Prime Minister Gordon Brown said on BBC Radio 4's Today programme that half a million of the EU citizens in Britain were Irish.