Complex sentence: The ACA changed its name to the "National Guard" under O'Duffy and adopted the uniform of black berets and blue shirts, using the straight armed salute, and were nicknamed "The Blueshirts".
Split into simpler sentences: The ACA changed its name to the "National Guard" under O'Duffy. They adopted the uniform of black berets and blue shirts, used the straight armed salute and were nicknamed "The Blueshirts".