Shire chief executive Flemming Ornskov said: "I believe that the combination of the two companies is in the best interests of shareholders and offers an opportunity to improve the lives of even more patients globally with rare and highly specialised conditions".
Shares in the FTSE 100 company climbed 4.6% to £40.34 a share by market close, though this was some way short of the £49 value under the terms of the offer, implying some doubt as to whether the deal would be backed by shareholders despite the unanimous recommendation of the board.
Takeda's financial advisers included Evercore Inc., JPMorgan Chase & Co. and Nomura Holdings Inc., while Shire received financial advice from Citigroup Inc., Morgan Stanley and Goldman Sachs Group Inc.
Takeda and Shire shareholders will each own about half of the combined company.
The costs of the debt funded offer also means Takeda will seek to reduce operational costs - by slashing jobs and cutting back on duplicated drug research.
Last year, Shire leased space in the former Bank of Ireland headquarters, on Baggot Street in Dublin and said it would boost staff numbers here to 300.
"We can now go after targets and diseases that we couldn't previously", said Plump, who oversees Takeda's research and development operations, which are based in MA. Shire's shares have soared 31 percent, giving the company a market capitalization of about $50 billion.
If it materialises, the deal would give Takeda greater access to the U.S. and European healthcare markets.
With few late-stage experimental drugs in its own pipeline, Takeda needs lucrative new therapies. Takeda was already Asia's largest drug maker.
Shire controversially moved its official headquarters from the United Kingdom to Ireland in 2008, helping reduce its tax bill. About 1,000 of its 5,000 US employees are in Deerfield.
Analysts and business professors recently predicted that the takeover will nearly certainly result in layoffs of several hundred Shire employees over the next couple of years - mostly among administrative employees whose duties could be filled by workers for Osaka-based Takeda. A company spokeswoman did not respond to a request for comment on how many of those employees are in IL.
Japanese drug makers have pursued growth overseas as sales slow at home. The Japanese company has a market value of just $33 billion, stoking fears about how much debt it will have to take on to fund the acquisition. Shire focuses on rare diseases, and the company's sale of its cancer business to Servier last month was viewed by some as a move to slim down in preparation of a merger.