Dara Ó Briain is somewhat unique among standup comedians in how much he interacts with the audience. Not just attacking people Dice-style or sparring with hecklers, he'll actually have full conversations with random people in the front row, or call out for answers to a question.
An important thing to note is that Dara does have solid improvisational skills. Check out about twenty-eight minutes into his 2008 special Talks Funny where he's able to almost seamlessly set up and deliver a multi-part bit based off of a woman unexpectedly shouting out "Energy!". But improvisation can be risky; unless you're Ross Noble, you need solid prepared bits to maintain a full standup set. Here's how he uses the audience to seem more improvisational than he actually is to help the show feel spontaneous and fun.
The part of Craic Dealer shown above (it starts at about fifty-eight minutes in for those of you reading after that video inevitably gets taken down for copyright infringement) starts with about two minutes of setup. The two pieces of technology chosen (computer, refrigerator) keep the audience thinking in a certain axis of technology (modern, electrical). If someone really wanted to be an asshole (NOTE: you shouldn't, hecklers suck), they could totally jump in with a suggestion like "lightning rod" or "not using lead pipes".
It's important to remember that this is his last show on this tour. He's had a few months to hone the details of the rehearsed parts, and he's done this bit multiple times. So when he responds to "microwave!" with "A microwave is the simplest machine you could think of!?", that's probably a prepared line he's used for other people who've shouted it out. And again, that isn't a knock on Dara, that's what comedians are supposed to do, they are supposed to have techniques to make a show funny.
Also notice how he subtly dismisses "kettle" after listing it as a good suggestion by focusing instead on "toaster". And when he asks for a "simple, non-electrical machine we use every day", there's really only one answer to that, which is naturally the one he's prepared for. To my ears it sounds like someone also shouted out "golf club", which again doesn't work in the bit.
By preparing the audience to think in certain ways and then selecting the answers that work best, Dara is able to make the show seem more free form than it actually is, and the show is better as a result.
(Dara if by any chance you're reading this, stop now)
This is one of the reasons I was disappointed with his most recent special, Crowd Tickler, which unlike his four earlier specials, contains no audience interaction in the beginning and very little throughout. (Only four times by my count, three of which are just different members of the audience saying "Yes." to something Dara says.) Maybe he's just low energy from having to host Mock The Week for sixteen worthless seasons.