Staying with the 1901 census at house 6 in Leitrim - More (Leitrim, Galway) 
LINKby 1911 there is living at house 9 in Leitrim More (Leitrim, Galway) a James COSTELLOE most likely the son with his family; and possibly Margaret BURKE's brother
He married a Mary DONOHOE on Nov 16 1904
James COSTELLOE of Leitrim died Oct 10 1905 aged 69; 
LINKMary COSTELLOE of Leitrim-more died Oct 26 1907 a widow aged 65 
LINKInformant on both their son James 
This couple had children as James COSTELLOE & Mary DWYER 
Mary Anne b Sep 07 1864 
cannot see other records immediatelyJohn Joseph b Sep 01 1869 
I think married Mary KANE on Dec 08 1897Catherine Josephine b Apr 20 1972 bap Apr 20 1872 
cannot see other records immediatelyBridget b Apr 28 1875 bap Feb 03 1875 
possibly Delia who married Denis KennedyMargaret Maria b May 09 1877 bap Apr 08 1877 
as shown on 1901 census as 23?James b Nov 20 1879 bap Jul 19 1879
as shown on 1901 census as 21?Dates of birth are from Civil Registers and Baptism from Church Records 
LINKUnfortunately the church records for marriage do not go up to the early 1900s
Also it was not unusual for baptisms to show dates earlier than the date of birth as children were baptised immediately but registered when they could! to save being fined for a late registration a recent date was sometimes given!
If these calculations are correct
Margaret's parents were James COSTELLOE & Mary DWYER b May 09 1877 
LINK