Complex sentence: De Valera explicitly instructed Ua Buachalla as Governor-General to keep a low public profile, and not to fulfil public engagements, which was part of de Valera's policy to make the office an irrelevance by reducing it to invisibility.
Split into simpler sentences: De Valera explicitly instructed Ua Buachalla as governor-general to keep a low public profile, and not to fulfil public engagements. This was part of de Valera's policy to make the governor-generalship an irrelevance by reducing it to invisibility.