By Padraic Halpin

DUBLIN (Reuters) - Ireland would have to consider taking steps to assist firms exporting into Britain if its nearest neighbor and largest trade partner votes to leave the European Union, Foreign Minister Charlie Flanagan told Reuters on Wednesday.

Ireland's economy is more vulnerable than any other in the EU if Britain opts to leave, and ministers including Flanagan have campaigned in Britain ahead of Thursday's referendum to urge Irish voters living there to vote "Remain".

Irish exporters would be the first to suffer if Brexit significantly weakened the pound against the euro and Flanagan said concerns had been raised in meetings with trade bodies during the campaign.

"It's too early for me to speculate as to the nature of any actions that we may take but we are fully sensitive to the challenge that lies ahead and the need to respond accordingly," Flanagan said when asked if there were any policy steps that could aid exporters.

"In the event of there being a vote to leave, we would be primarily focused on strategic national Irish interests across a range of issues, primarily economic and trade, so we have prepared a plan across a range of departments."

Around 1.2 billion euros ($1.35 billion) of goods and services are traded between the United Kingdom and Ireland each week with Irish farmers and food producers, major UK suppliers, particularly vulnerable.

Flanagan, serving a second term as foreign minister after his Fine Gael party returned to office last month, also predicted any withdrawal negotiations could take much longer than the two-year period laid down in EU law.

Of most concern to Ireland during such a process would be the impact on Northern Ireland, which has the only land frontier between the United Kingdom and the rest of the EU.

While pro-Brexit campaigners say a vote to leave would not endanger a common travel area that predates both countries' entry into the EU in 1973, Flanagan said he could not see why the reintroduction of a hard border would not be considered.

Given the prominence of immigration as a campaign issue, he said, it would seem logical that Britain would be inclined to take some action if it was outside the EU. In that case, Dublin was concerned that the negotiation on a form of border would be done at EU level and therefore not entirely under its control.

The fear for many is that any new border restrictions could endanger an 18-year-old peace agreement between Catholic Irish nationalists seeking a united Ireland and their Protestant rivals who want to keep Northern Ireland British.

"At best, I would point to a period of uncertainty that would affect the great progress that has been made (in Northern Ireland) in recent years," Flanagan said in his office in Dublin's government buildings.

"And in terms of the peace process, any re-introduction of border controls, either in terms of custom controls for trade or in terms of security, that could well pose us with a difficult challenge."

(Editing by Catherine Evans)