The best actress for Mrs. Conlan would be someone who can access an emotionally frigid disposition. Mrs. Conlan's high strung and "hyper" condition prevents any real emotions to enter. She uses barriers to prevent real emotional connection between she and her son and husband. Being able to construct an emotional dimension difficult to imagine is perfect for an actress like Meryl Streep or Glenn Close. Both actresses have demonstrated themselves to be ideal at achieving rare emotional dynamics. Both actresses thrive on depicting characters that possess emotional depth. Their real gift is being able to depict anyone who might be perceived as monolithic in a complex manner. In the body of work that both actresses possess, they can depict characters who are emotionally rich or challenging. The idea of depicting a character that "is impossible to relax" is perfect for both Glenn Close and Meryl Streep.
If age is not an issue, I think that an actress like Ellen Burstyn or Kate Winslet would be alternate choices to depict Mrs. Conlan. Ellen Burstyn might be a bit outside of the range of Mrs. Conlan, but her roles throughout her career as emotionally rich mothers in films like The Exorcist and Requiem for a Dream make her a great choice to consider for Mrs. Conlan. Finally, I think that Kate Winslet has amazing range. She can depict characters who run the gambit and she might enjoy taking on a challenge such as Mrs. Conlan. Kate Winslet has defined her career by taking choices of characters that are divergent and ecclectic. This might be one reason why Kate Winslet could be a choice to portray Mrs. Conlan.