He was born 17 November 1897 in Knockaderry, County Limerick, one of two sons and a daughter of David O'Brien, a creamery manager, and Kathleen O'Brien (née Casey). He was educated at Ahalin national school and then at the Redemptorist College at Mount St Alphonsus, Limerick city, but was prevented from going to university by bouts of ill health.[3]
On leaving school he worked in the creamery managed by his father for a number of years. By 1917, he had become involved in the independence movement as a member of Sinn Féin and later, of the West Limerick brigade of the Irish Republican Army. He was involved in the republican courts in Limerick and took the anti-treaty side in the Irish Civil War.
He was deeply involved in the Conradh na Gaeilge, which he also joined in 1917, having been influenced by Fr Tomás de Bhál. In 1920 Ó Briain was appointed Conradh na Gaeilge organiser for County Limerick, and from 1925 he served in that role for all of Munster province, founding numerous branches of the league. From 1928 to 1932 he served as general secretary of the Conradh na Gaeilge and also intermittently edited Fáinne an Lae.[3]