Meet your new mother-in-law, Beiv (Kate Mulgrew): she’s Ireland’s “great feminist artist” who lives on a remote island off the coast of West Cork, has an acerbic wit and an imperious manner, a penchant for knocking down the walls of her house, a strained relationship with her son, and may have murdered her husband. Oh, and she didn’t know her son was married until you showed up with him from San Francisco. This is the situation that 23-year-old Bonnie (Ayana Workman) finds herself in at the outset of Nancy Harris’s overstuffed mystery The Beacon, which was commissioned by Druid Theatre Company and now arrives at the Irish Rep after a 2019 run at the Gate Theatre in Dublin.