Complex sentence: This appeared to show that there was a substantial Irish republican support base within Northern Ireland, but as the mainstream Nationalist Party decided not to take part in the election, its supporters had voted for Sinn Féin instead.
Split into simpler sentences: This appeared to show that there was a substantial Irish republican support base within Northern Ireland. However as the mainstream Nationalist Party had decided not to take part in the election, its supporters had voted for Sinn Féin instead.