The missing item on Angela's prescription is for lithium. Lithium is frequently used to treat mania that is part of bipolar disorder (manic-depressive illness). It is also used on a daily basis to reduce the frequency and severity of manic episodes.
The Cronins Pharmacy where Char goes to fetch a missing prescription for her mother is in fact a real shop of the same name in the Edenmore North eastern district of Dublin.
When Char walks home from school she passes a playing field amidst a housing estate complete with a large stone boulder. This is probably part of a megalithic remains their being many such structures around Dublin.