The Riddles of Auld Irishtown #1

“What is there to be said?” Said the man who is nearly dead. 

“There’s just too much to say,” Say he. “So many facts in the way, for I only want you to know. Want you to feel how real my life was back when. All of it a symbol. So many! So nimble! The rhymes, the riddles so elegantly allied. But how can you know? How can you find? Well most certainly you cannot, without knowing the mysteries of histories from the O’Reilly’s of Breifne and the O’Connor’s of Connacht, Leinster, Munster & Ulster too. For who can you see, in the Auld Irishtown trilogy as a representative of the ancient glories of history?” 
 
First Riddle: 

T’was 1913 when Brooklyn fiefdoms reigned,
Dinny took power, dockboss jobs entertained,
Those loyal were given their place
While others were ignored, debased, 
Diarmait of the Foreigners has his home in this lore,
For who among the many

Would search long and plenty
To regain his kingdom, start a war? 

Answer – (not just the character’s name, but an explanation)