DUBLIN — Sinn Féin, the main opposition party in Ireland, has lost out to a smaller left-wing rival in Saturday’s by-election result in Dublin — and Sinn Féin leader Mary Lou McDonald is feeling the heat. A political newcomer, Daniel Ennis of the Social Democrats, romped home in Dublin Central following Friday’s vote to fill a parliamentary seat previously held by former Finance Minister Paschal Donohoe, who quit last year to join the World Bank. Sinn Féin’s candidate, Janice Boylan, came