Eolais mac Biobhsach (alias 'Irish: Eolus' "knowledge", anglicized "Wallis", "Olis" or "Olus" fl. AD 900) was a chieftain of 10th century Gaelic Ireland. He is noted as the first "full chieftain of the Conmaicne" of present-day south county Leitrim, and parts of west County Longford. His descendants are known as "the Muintir Eolais".