After a promising Army career in the American Civil War he joined forces with the Fenians and later established an Irish colony in Nebraska where today the city of O'Neill, Nebraska stands as a memorial to his endeavours on behalf of the Irish communities in the United States of America.
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Another native of Tassan, Clontibret was Senator John Brennan, a Fianna Fáil member of Seanad Éireann from 1960 until 1977 and a member of Monaghan Co Council from 1942 until 1974.