Up to 330,000 residents of Ireland could have an influence on the outcome of Britain's decision on European Union membership as they will be entitled to a postal vote in the upcoming referendum.

UK citizens living in Ireland, as well as Irish citizens who lived in the UK during the past 15 years and registered to vote, can cast a ballot in the referendum, which could take place as early as June.

A campaign to encourage voter registration by all those living outside of the UK is under way.

The last Irish census in 2011 showed that 288,627 people listed their place of birth as being in the UK.

Added to that, Irish people who have lived for a period in the UK since 2000 and who voted there in elections and referendums during that time may also be entitled to vote in the referendum.

Locations

The British population in Ireland is concentrated in rural Co Cork, Dublin and the commuter belt, Kerry region,

Galway

, Mayo and

Donegal

.

Some 106,000 people living outside the UK are registered to vote there – three times the number that was on the register ahead of the general election in 2010.

However, this number represents just a fraction of the 5.5 million UK citizens estimated to be living overseas.

More than half a million Irish-born people living in the UK will be eligible to vote, along with residents from Commonwealth countries and Malta.

Citizens from other EU states resident in the UK will not be able to vote.

Potential voters can check their eligibility to vote in the referendum and can register to vote online.