thanks very much for the reply,
Yes, I believe that's the family in 1911. Here they are in 1901
http://www.census.nationalarchives.ie/pages/1901/Antrim/Clifton/Hazelnut_Street/961830/Yes, I think it's likely that the person my mother referred to as Etta, is the same person as the Maggie listed in the census.
I've searched on the IFHF website, Emerald Ancestors but can't find the right record on those.
Thanks for pointing me at the LDS website, I just tried the new site at
https://beta.familysearch.org, it lists many more possible matches but has insufficient information to confirm, or otherwise, the correct entry.
The Following all look possible
Name Margaret Hamilton
Registration District Belfast
Event Type BIRTHS
Registration Quarter and Year Jul - Sep 1905
Volume Number 1
Page Number 366
Name Margaretta Hamilton
Registration District Belfast
Event Type BIRTHS
Registration Quarter and Year Oct - Dec 1906
Volume Number 1
Page Number 373
Name Margaret Hamilton
Registration District Belfast
Event Type BIRTHS
Registration Quarter and Year Oct - Dec 1906
Volume Number 1
Page Number 166
Name Margaret Hamilton
Registration District Belfast
Event Type BIRTHS
Registration Quarter and Year Jan - Mar 1905
Volume Number 1
Page Number 208
Name Maggie Jane Hamilton
Registration District Belfast
Event Type BIRTHS
Registration Quarter and Year Oct - Dec 1906
Volume Number 1
Page Number 174
Name Maggie Hamilton
Registration District Belfast
Event Type BIRTHS
Registration Quarter and Year Oct - Dec 1905
Volume Number 1
Page Number 300