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

Paul Flynn said the watchdog set up after the expenses scandal had failed to restore public confidence.
But the Newport West MP said his views were being used as a "stunt" in the ongoing row within Labour.
The Independent Parliamentary Standards Authority (Ipsa) said it was currently consulting on the rules.
Labour distanced the party from Mr Flynn's comments, emphasising they were not Labour policy.
In a blog post, the shadow Welsh secretary described Ipsa as a "bureaucratic ornament" which he said had taken "minute control of claims large and small".
He said it had failed to restore faith in the expenses system, which was time-consuming and robbed MPs and their staff of their "most precious possession - time".
He wrote: "Our reputation has sunk from rock bottom to subterranean. Financial scandals have continued in both Houses with toe-curling regularity.
"The public are still convinced that MPs use the system for own ends."
Mr Flynn suggested a new system could be based on an allowance calculated on average expenses, based on MPs' distance from Westminster, and could be paid automatically.
He claimed MPs would accept such a change, even if it meant they were worse off, because they would be liberated from the "tentacles of tedious bureaucracy".
Speaking on BBC Radio's Good Morning Wales programme, Mr Flynn suggested that he originally outlined his views in a document written nine months ago "as a backbencher".
He told the programme: "Jeremy Corbyn knows nothing at all about it and this is now being used as a stunt by people in this mad battle we have in our own party where we are tearing ourselves apart.
"This is nothing to do with expenses, except that Ipsa is being looked at at the moment and we've been asked for ideas. I've put forward ideas that I published nine months ago."
Delyn Labour MP David Hanson said Mr Flynn's suggestions were "wrong".
"The public expect us to have accountability and transparency on the money that we spend on behalf of them to provide constituency staff, constituency offices and the services that we provide," he said.
"I have no problem whatsoever as a member of parliament with my staff in completing Ipsa's requirements and filling in forms to account for that money."
A spokesman for the Labour party said: "The public rightly demands the trust of their politicians and Labour therefore remains committed to the Independent Parliamentary Standards Authority which Labour established to ensure MPs do not set their own pay and conditions."
An Ipsa spokesman said: "Ipsa is currently consulting on rules for MPs six years after they were introduced and is seeking views from members of parliament and members of the public."
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.
[NEXT_CONCEPT]
MPs should be given an allowance instead of having to claim for expenses, Labour's shadow Commons leader has said.