Thanks Brian,
I found the 1901 info (Ancestry.uk has Ballaugh in Anglesea).

In 1891 Knockould, Ballaugh there are:
Catherine Cowle lodger 45 single In receipt of Parish relief b. Ballaugh
Margaret Cowle lodger 10 scholar b. Ballaugh

In 1851 there is a seven-year-old Catherine Cowle in Ballaugh Bk 2 Page 42 Entry 4, with James (37), Elizabeth (35),Edward (2), Elizabeth (8), John (6), (William 9 months)and Thomas (4).

In 1861 this family, without Catherine, is at Carmodil beg? in Ballaugh RG9/4411. James' occupation is Carrier. New addition is Isabella (2). Catherine (17) is a domestic servant at Squeen Mill? Ballaugh

In 1881 same address (Carmodyl witha "y").
Elizabeth (29) daughter is now Watterson and a widow. John and Catherine are not at this address.
I cannot locate Catherine (or Kate) in either the 1871 or the 1881 census. Possibly she was off the island.
The Ballaugh Parish christening records on the IGI are pretty slim after 1875, so Maggie is unlikely to appear there.