A Treble, Two Doubles, And Three Singles -- Bringing-up twins has its problems but when you have two sets of them, triplets, and three other children, like the Donnellys of Sheerness, then you have a handful. Here Mrs. Donnelly, Mary (12) and Dolores (11) are nursing the six-month-old triplets, Elizabeth, Eileen and Cecilia-they were runners up in a local baby show-while Mr. Donnelly holds Meave and Monica, the 18-month-old twins. In the front Rosaleen and Catherine, aged five, sit with three-ye