The women would have their tea at six o’clock and she would be able to get away before seven. From Ballsbridge to the Pillar, twenty minutes; from the Pillar to Drumcondra, twenty minutes; and twenty minutes to buy the things.
Based on having read the entire story, she seems like a character who must be very particular and have a lot of structure/order in their life, to the point that any deviation is extremely upsetting.