Sean Lemass was an Irish Fenian, who participated in the Easter Rebellion of 1916.
In 1916, Sean Lemass' sister, Maggie, had a romantic interest in Indiana Jones. Sean was initially very skeptical towards the young American, but eventually Sean and Indiana became friends.
Lemass was later active in the Irish War of Independence. In 1959, Sean Lemass became the head of the Irish republic.
Sean Lemass was portrayed by Darragh Kelly in The Young Indiana Jones Chronicles. Although the historical Lemass had several siblings, his sister Maggie is an invention for the production.
As Taoiseach, head of the Irish government, Sean Lemass introduced economic reforms that encouraged Ireland's industrial growth and brought in foreign investment.