Britons trying to hang onto EU citizenship have inundated Ireland's embassy in London and post offices in British-run Northern Ireland with passport enquiries and requests for application forms, the Irish foreign office says.
Post offices ran out of forms and the embassy fielded more than 4000 passport enquiries compared to the 200 a day it usually gets, a diplomatic source told Reuters on Monday.
Anybody born in the Irish Republic or Northern Ireland, or with an Irish parent or grandparent, is entitled to an Irish passport - about six million people living in Britain.
"Following the UK referendum, there has been a spike in interest in Irish passports in Northern Ireland, Great Britain and elsewhere," Irish Foreign Minister Charlie Flanagan said.
Flanagan warned that a surge in applications would place significant pressure on turnaround times at passport offices and could affect those with imminent travel plans.
A member of Northern Ireland's largest Irish nationalist party, Sinn Fein, called on the government in Dublin to open a passport office in Belfast after post offices ran out of forms and were unable to meet demand until more arrived.
Even pro-British lawmaker Ian Paisley, the son of the firebrand preacher-politician of the same name who for decades cried "No Surrender!" to Catholic nationalists' desire for closer ties with the Irish Republic, advised constituents to apply for a second passport.
Northern Irish citizens can hold both an Irish and British passport.
Elsewhere, views are mixed among the estimated 400,000 British expatriates entitled to dual citizenship if they have lived in France for five years or have a French spouse.
Some Britons are applying for Belgian passports as long-time residents of Belgium or applying to the countries of their European spouses.
In London, where the population of 8.6 million is among the most cosmopolitan in the world, younger workers who mainly voted for Britain to remain in the EU were also mulling their options.
