Original: De Valera and many other members of the republican movement objected to Ireland's status as a dominion of the British Empire and to the symbolism of having to give a statement of faithfulness to the British king to this effect.
Simplified: De Valera and many other members of the republican movement did not want Ireland to be a dominion of the British Empire. They did not like the symbolism of having to give a statement of faithfulness to the British king.