The company said the lawsuits filed on behalf of US investors were "without merit"
Elan is the latest company to fall victim to increased investor suspicion of off-balance-sheet vehicles, the accounting mechanism which allowed Enron to fudge its true figures and which ultimately led to its downfall.
Shares in the company - once Ireland's biggest by market capitalisation - have plunged on news of the lawsuit.
Without merit
Milberg Weiss Bershad Hynes & Lerach initiated proceedings in a southern California federal court.
In a separate filing, Wolf Haldenstein Alder Freeman & Herz said it had filed a class action lawsuit in a New York federal court.
The Elan accounting system allows the company to shift some of its research and development partnerships off its income statement.
At the same time, it allows the company to record technology licensing fees as contract revenue.
Confusion also exists because it filled accounts according to US and Irish accountancy rules, resulting in two different sets of figures.
The suspicions grew when the company warned earlier this week that this year's profit could be as much as 34% lower than expected.
Last summer, the company had a market capitalisation double the size of any other Irish company.
On Tuesday, it had fallen to fourth in the list of Ireland's largest firms.
Investor suspicions
Attention has not just focused on Elan, but also GECC and Tyco, as investors became more suspicious in the wake of the Enron collapse.
"What you have is the exposure of problems that were covered up by the prosperity and the internet bubble of the late 1990s," said Fred Isquith, a lawyer at Wolf Holdenstein in New York.