What child-development types like to call “unstructured play” is anything but: The games kids play, such as those the young Irish lads Mojo and Mickybo come up with to fill their days, tend to be highly ordered affairs with strict sets of rules. The identities that the two heroes of Owen McCafferty’s play adopt (Batman and Robin, Butch and Sundance) while evading the clutches of Belfast’s stern adults and menacing street toughs are sharply defined, as well—so sharply that you’ll forgive the play’s desire to say something important about What Happens When the Adult World Intrudes With Rules of Its Own. Christopher Dinolfo’s Mojo, we learn, is from “up the road.” Mike Innocenti’s Mickybo is from “over the bridge.” The play’s set in Northern Ireland in 1970. See where this is going? But while you’re steeling yourself for the message to kick in, the Keegan Theatre’s fast, funny, and energetic production will likely win you over. Dinolfo and Innocenti play very young boys who evince a wide-eyed understanding of the world, but they’re both disciplined actors and keep the evening sugar-free. Director Eric Lucas sends the pair caroming across Theatre on the Run’s compact stage, and they work up a sweat miming shootouts and fistfights. Mojo Mickybo’s at its sharpest, however, when it lets its titular characters occupy themselves with less raucous pursuits, as when they debate the relative merits of Spider-Man vs. Batman. (Spidey, they decide, would be much more useful in Belfast, because he could web up bombs to keep them from exploding. The Dark Knight, in their considered opinion, would do fuck-all.) The real treat is watching Dinolfo and Innocenti tackle the world that hovers around the boys, rotating through a diverse cast of characters that includes their respective mums and das, a dipsomaniacal war veteran, and a pair of lumbering brutes with the satisfyingly evocative names of Gank and Fuckface. Both actors have fun with this variety show, but Dinolfo is particularly deft and economical; he doesn’t so much shift into a different persona as click firmly into place. It’s a thing to see: I found myself thrilling to a particular tilt of the actor’s head, because noticing it meant knowing that Mickybo’s mother—the play’s go-to laugh-getter—was about to speak again.