A classically trained cellist, Rankin performs on a carbon fiber cello in pubs because beer spills pose a risk to her wooden instrument. She became hooked on Irish fiddle music through the work of Martin Hayes, an Irish virtuoso steeped in the sweetly lithe tradition of East Clare fiddle playing. “He was my gateway drug,” she said.

Hayes is a member of the Gloaming, a supergroup that unites some of the biggest names in Irish music and plays to capacity crowds in concert halls around the world. But, as Hayes said in a phone interview, concerts cannot replace sessions.

“If they weren’t here, the world of Irish music would be very disconnected,” he said. “It’s one thing to go and see a concert, but it is very instructive to have a continuous investigation with the music on a weekly basis.”

When in New York, he sometimes joins in the session at the Dead Rabbit near Wall Street on Sundays, or plays alongside DeMarco at 11th St. Bar. Yes, he says, background noise and chatter can mean that some of the nuances of the music are lost in a pub. But, he added, “musicians also like the fact that it’s not a gig and they don’t have to deliver. And one of the key elements in the creation of any music is the ability to be relaxed.”

At first glance, a session looks laid-back, spontaneous and open. Yet there are hierarchies underpinning both the citywide scene and each individual session. Some are bloated with beginners, compromising musical standards. And establishing your own session is a process.

Daniel T. Neely, a banjo player and ethnomusicologist (his expertise: Jamaican mento music), who said he was “press-ganged” by Moloney into playing in an Irish group, has published a set of rules for newcomers regarding egos and etiquette. No. 8: “There is a good chance that an eyebrow raised in your direction is consequential.”