Sheehan and The Lakota People's Law Project participated in legal cases related to the Dakota Access Pipeline protests.

[5] Among the 30 defendants named were Iran-Contra figures John K. Singlaub, Richard Secord, Albert Hakim, and Robert W. Owen; CIA officials Thomas G. Clines and Theodore Shackley; Contra leader Adolfo Calero; Medellín Cartel leaders Pablo Escobar and Jorge Ochoa; Costa Rican rancher John Hull; and former mercenary Sam N.

Avirgan complained that Sheehan had handled matters poorly by chasing unsubstantiated "wild allegations" and conspiracy theories, rather than paying attention to core factual issues.

[12] Sheehan has spoken publicly about unidentified flying objects (UFOs) and alien visitation, and has served as counsel for Harvard University psychiatrist John E. Mack[13][14][15] as well as Steven Greer's Disclosure Project.

[16] He represents Luis Elizondo, the former director of the Advanced Aerospace Threat Identification Program in a case against the United States Department of Defense.