Within the past two weeks the company registered a business in Ireland and a spokeswoman confirmed the group is seeking office space in the capital. She said the office would be mainly administrative, employing few people. She added that it was not being established to avail of Ireland's low corporate tax rate.

Biogen Idec and Elan recently received limited approval from the US Food and Drug Administration for the sale of Tysabri. Links to a potentially fatal brain disease led the agency to restrict its availability to patients who have not responded to other MS treatments. Biogen Idec sells another MS drug called Avonex.

Tysabri will be the most expensive MS treatment on the market when it becomes available next month. About 2.5 million people worldwide have multiple sclerosis.