Sentence:  The absolute intractability of the Irish question for successive waves of the English liberal consensus remains with us, at least in the diminuendo of Ulster, to this day; but revolving round this <WSD>"primum mobile</WSD>" of colonialism (and recall: Ireland was put to the sword by parliament, not monarch) are all the other crystal spheres of the expansive English cosmos. 
Meaning of the word marked with <WSD> tags:  The person or thing that is the main impetus for some action; a driving force.