Complex sentence: The President of the Republic, Éamon de Valera, realising that a republic was not on offer, decided not to be a part of the treaty delegation and so be tainted by more militant republicans as a "sellout".
Split into simpler sentences: The President of the Republic, Éamon de Valera, realised that a republic was not on offer. He decided not to be a part of the treaty delegation and so be tainted by more militant republicans as a "sellout".