Thank you, Donna. So Margaret Costain nee Quirk was definitely alive at the 1841 census, and these are the youngest MQs:

QUARK Margaret 4 Rus 1465/12 4 2
QUIRK Margaret 3 Ger 1463/7 10 13
QUIRK Margaret 2 Pat 1465/10 23 12
QUIRK Margaret 1 Bra 1463/5 22 6
QUIRK Margaret 6 mos Dou 1465/7 9 13

In 1881, she's "b Rushen", but then so is her husband John, and we know that he was actually born in Arbory. So there's no way of knowing which MQ she is. My money's still on the 4 yr old from Rushen.

Jean