I actually thought there was a particular point to here being a dominatrix. She and Sherlock are mirrors of each other - and for each (both narcisissts I think) they love getting the better of people, love dominating if you will if in their work. The difference I would say would be that Irene actually gets people's consent, while Sherlock doesn't (and we see the negative effects of this for example in his interaction with Molly) .

And I thought the point of stating she was a lesbian, and John is straight, was to make the point that her/their connection to Sherlock wasn't sexual!

And seriosuly she can sit naked on my lap anyday!