Clarke’s father was a soldier in the British army. Clarke’s links with Clan na nGael in America, where he lived for many years, copper-fastened his importance to the revolutionary movement in Ireland. He served 15 years penal servitude in England for his role in a bombing campaign in London, 1883-1898. He was Treasurer of the Irish Republican Brotherhood and a member of the Supreme Council from 1915. The first signatory of the Proclamation of Independence through deference to his seniority in the nationalist movement, Clarke was with the group that occupied the GPO on Easter Monday.
Bhí athair Thomáis Uí Chléirigh ina bhall d’airm Shasana. Chuir an ceangal a d’fhorbair an Cléireach le Clan na nGael fad is a bhí sé ina chónaí i Meiriceá go mór leis an ról lárnach a bhí aige i ngluaiseacht réabhlóideach na hÉireann. Gearradh cúig bliana déag de dhaorphionós air de bharr an bhaint a bhí aige le feachtas buamála i Londain, 1883-1898. Bhí sé ina Chisteoir ag Bráithreachas Poblachta na hÉireann agus bhí sé ar an Ardchomhairle acu ó 1915 ar aghaidh. Bhí an Cléireach ar an mbuíon a ghabh seilbh ar Ard-Oifig an Phoist Luan Cásca agus ba eisean an chéad duine a chuir a ainm leis an bhForógra, in ómós dá stádas i measc náisiúnaithe na hÉireann.