Barry Keoghan would love to celebrate his newfound stardom with his late mum. The ‘Banshees of Inisherin’ actor, 31, has seen his level of fame soar since he starred as scheming Oxford University student Oliver Quick in Emerald Fennell’s popular ‘Saltburn’ drama, but said he is stricken by a “loneliness” he thinks comes with fame. He added to the February issue of GQ about how he fills the void by thinking of his mum, who died from drug addiction when he was 12: “When I’m isolated (what do I think of)? “Obviously, my mother. My mother, always. She’s many years passed now, but I always think ab…