Prior to the Rising, Ceannt was an employee of the Dublin Corporation. He was a co-founder of the Irish Volunteers, partaking in the successful Howth gun-running operation of 1914. His involvement in republican activities was complemented by his interest in Irish culture, specifically Irish language and history. He was an accomplished uilleann piper. As commander of the Fourth Battalion of Irish Volunteers during the Rising, he took possession of the South Dublin Union (modern-day St. James’s Hospital).
Roimh an Éirí Amach, bhí Ceannt fostaithe ag Bardas Bhaile Átha Cliath. Bhí sé ar dhuine de bhunaitheoirí na nÓglach agus ar dhuine den dream a thug na gunnaí isteach go Binn Éadair in 1914. Chomh maith le bheith páirteach i ngníomhaíochtaí poblachtánacha, bhí an-spéis aige i gcultúr na hÉireann, go háirithe an Ghaeilge agus stair na hÉireann. Píobaire uilleann tréitheach ab ea é. Agus é ina cheannfort ar Cheathrú Cathlán na nÓglach le linn an Éirí Amach, ghabh sé an South Dublin Union, an áit a bhfuil Ospidéal San Séamas sa lá atá inniu ann.