(New throughout, adds details on judge's ruling)
By Jan Wolfe
Oct 16 (Reuters) - Shares of pharmaceutical company Allergan Plc dropped on Monday after a ruling by a U.S. judge invalidated patents on its dry eye medicine Restasis on the grounds that the patents cover obvious ideas.
Judge William Bryson issued the ruling in federal court in Marshall, Texas, in a longstanding dispute between Allergan and generic drugmakers led by Mylan NV and Teva Pharmaceutical Industries Ltd.
The ruling could enable the generic drug companies to sell their own versions of Restasis, which generated around $1.5 billion in sales for Allergan last year and accounted for more than 10 percent of the company's revenue.
Allergan's stock price dropped about 5 percent on the news.
An Allergan spokesman did not immediately return a request for comment.
The patents at issue were the same ones Allergan transferred to a Native American tribe in an effort to protect them from administrative review. The company said it would not invoke the tribe's sovereign immunity in federal court. (Reporting by Jan Wolfe; Editing by David Gregorio)