Dempsey had been among those who had risen with United Irishmen against the British Crown and the Protestant Ascendancy in the early summer of 1798. He had taken part in the Battle of Vinegar Hill and had suffered a severe shoulder wound at Hacketstown. In 1799 he stood trial on three counts: the murder of two loyalists, killed after Vinegar Hill; the murder of two soldiers in the British army at Ballyellis; and rebellion in general. After an outstandingly scrupulous hearing at Wicklow (north of Wexford), Dempsey was found not guilty of both murder charges but was sentenced to transportation for life as a rebel.