Blood in the Water by Michelle St. James

Nolan Burke is in deep sh*t.
After the failed attempt to end Seamus O’Brien’s rule over Boston’s criminal underworld, Nolan and the Syndicate are back to square one, trying to find a way to take control of the territory from Seamus and a cadre of former IRA fighters. Nolan is willing to help the Syndicate, but only to protect the love of his life, Bridget Monaghan, from the debt she owes Seamus, a task that becomes more difficult as the turf war becomes more perilous.

Bridget Monaghan is walking a tightrope over a crocodile — and his name is Seamus O’ Brien.
Bridget is trying to keep her head down and her sick brother alive while the Syndicate plots the takeover of South Boston’s criminal organization, a takeover that will eliminate her debt to Seamus O’ Brien. As if that’s not enough, her renewed relationship with Nolan Burke is built on a lie — or the truth she hasn’t told him about the reason she left him the first time around.
But it’s hard to keep secrets when everyone has something to lose, and soon Nolan and Bridget are on the run as everything is revealed. When a hideaway meant to offer protection becomes the site of a deadly game of cat and mouse, Bridget is forced to defend herself and her family while Nolan races the clock to save their lives — and their love.