Finngarick by Victoria Danann

Torn Finngarick despises the phrase ‘bad boy’.
Sheridan O’Malley is on her way to becoming a Black Swan legend.
Dublin is about to become a lot less demon-friendly.
When ex vampire hunting knight, Sir Torrent Finngarick, is hired by D.I.T., he’s partnered with one of a pair of near-feral, New Forest elf twins who also happens to be his mate. Unfortunately Sher O’Malley made a pact with her twin when they were children that they would never accept a mate.
After rigorous training with Black-Swan-friendly demons and Black Swan knights emeritus, they’re assigned to Dublin because there’s an interdimensional stream portal somewhere in the vicinity of Trinity College and Temple Bar that’s been causing havoc for centuries.
Just when Sher is succumbing to the inevitable pull of mating, she and Torn chase a trespasser through the portal underneath St. Patrick’s. The wild redheaded beauty catches the demon’s eye. And disappears.