Kraft Foods has agreed a deal to buy Cadbury for around £11.9 billion breaking a four-month impasse between the firms.
The deal, which consists of 500p of cash and 0.1874 new Kraft shares, won the unanimous backing from the Cadbury board.
Trade union Unite has opposed a takeover by Kraft, saying the US company would need to lay off tens of thousands of workers to achieve its savings targets in a deal.
The Irish branch of the union, which also represents workers at Cadbury in the UK, said the sale of Cadbury to Kraft is a "very sad day" for Irish manufacturing and the company.
It is concerned for the future of the company and its workers remained, claiming workers and key shareholders were excluded from consultations on the takeover.
"A successful, iconic, independent brand will now be owned by a giant company with massive debt," said Jimmy Whelan, European co-ordinator for Unite members in Cadbury.
"We have very real fears about how Kraft will repay its debt, particularly as it has ratcheted it up still further in order to purchase Cadbury. Whatever good intentions Kraft may have towards Cadbury's workforce, the sad truth is there will be an irresistible imperative to pay down their debt, and this raises real fears for jobs and investment in this country."
The company employs almost 7000 workers in the UK and Ireland. It has had manufacturing operations in Ireland for more than 75 years, with over 1,200 workers at three plants in Coolock and Tallaght in Dublin, and Rathmore in Kerry.
The union said it would be seeking meetings with management of both Kraft and Cadbury to obtain guarantees over jobs and sites in the UK and Ireland.
Cadbury had rejected Kraft's previous £10.5 billion hostile takeover bid since a brief conversation between the two sides in late August.
But with a deadline looming to raise its bid, Kraft chief executive Irene Rosenfeld held talks late into the night in central London with Cadbury chairman Roger Carr and chief executive Todd Stitzer.
Kraft had been expected to raise its bid by the deadline set by the UK Takeover Panel and many Cadbury investors said they would not contemplate an offer below 800p to 850p per share. Earlier yesterday, major Cadbury shareholder Standard Life said Kraft needed to bid over 900p per share to get its support.
Cadbury, the maker of Dairy Milk chocolate, had said it sought to remain on its own as long as Kraft's offer undervalued its businesses.
A deal with the US food group would mark the end of independence for a British institution built 186 years ago when John Cadbury opened a shop in Birmingham selling tea and cocoa.
"I identify them with plastic cheese on hamburgers," said Felicity Loudon, a descendant of Cadbury's founding family, when asked about Kraft in November.
Additional reporting - Reuters
