So Irene throughs a masquerade ball/play party(because whats the point of being a rich dominatrix if you cant throw a masquerade ball). And Eurus sneaks in. And catches Irene's attention.

So they flirt and Irene takes her back to her room but evem when clothes come off the masks stay on. That's the rules.

Years later Irene has changed her name and has a semi normal life as a romance novelist(most of it autobiographal) and she meets a rather cute shy thing(think "E") and they go out to cute seaside cafes and antique shops. But Irene doesn't pursue a relationship because of her enemies and Eurus thinks it's obvious they are in a relationship.

Then Mycroft shows up. And Irene thinks he's after her. Then he starts asking for Eurus. Irene doesn't know who he is talking about but then Eurus walks in(she's wearing jeans, flip flops, and a pink t-shirt) and says, "Its about time you show up. You're getting slow brother mine".

And Mycroft and Irene both stare in shock. Irene thinks she should have known she would fall fir another holmes.

Anyway Irene isn't gonna let Mycroft take away the one friend she has. And Eurus is aloughed to stay under probation with Irene.

A favorite of mine!

Normally I have some trouble getting into pairings that never appear on screen together, but as soon as I thought of the trouble these two would get up to together, I was sold. 

Eurus is brilliant but a bit lacking in the social skills department -- as she complains to Sherlock, she “never had a friend.”  Meanwhile Irene has half of London eating out of her hand.  Eurus’s only sexual experience was gruesomely violent and unsatisfactory...  Irene is an expert in the skilled deployment of pain and humiliation, who knows how to deliver exactly what her partner wants.  Irene likes to misbehave -- Eurus could take that inclination to a whole new level.  Irene is clearly bored with Kate and needs someone unpredictable, who will keep her on her toes.

There, you see her, sittingthere across the way She don't gotta lot to say, butthere's something about her And you don't know why, butyou're dying to try You wanna kiss the girl...

In a Kingdom.

Eurus is hiding her relationship from her brothers. But when they both start flirting with her girlfriend at a party she's had enough. She glares at both of her brothers and then power walks to Irene and gives her a very passionate kiss.

When Eurus pulls back Irene winks at the boys and says that she needs to go.

Irene runs dance night at Eurus' bar. She starts staying over when Eurus closes up. Then she starts staying the night with Eurus on dance night. Then she's spending all her free time at the restaurant or Eurus' house. They kinda just fell from friends to friends with benefits to couple without noticing. Until one night they are laying together in bed and Irene asks Eurus if she wants to get married. She says it doesn't matter it's just a piece of paper but she would like her to meet her family for Christmas.