This is what we have seen already- note no place of residence transcribed for Margaret - assumed perhaps both Galway.
http://www.theshipslist.com/ships/australia/grandtrianon1855.shtml
The same records transcribed for Find My Past are in the name of Gahagan and Margaret's place of origin shows Galway.
It could just be an interpretation of the records which is transcribed and unless the original record is seen you wouldn't know.
Research on Thomas and his wife in South Australia and sources of information including information from
BISAhttp://www.rootschat.com/forum/index.php?topic=768182.0The survival rate for passenger lists in South Australia is poor.
Sources for information on the website -
State Library South Australia,
Official passenger lists, mainly of Immigrants arriving in South Australia under United Kingdom assisted passage schemes, 1847-1886
GRG 35/48 (formerly ACC 313);
Sydney Shipping Gazette;
South Australian Register;
The South Australian Government Gazette;
GRG 35/48/2 Crown lands and Immigrant ships papers;
Biographical index SA 1836-1885 (the B-index column indicates individuals who may be found in that index, with corresponding reference number for further research) ;
FreeBMD;
1851 UK census
From SA Records
https://www.archives.sa.gov.au/content/our-catalogueSeries Id:
GRG35/48 Records relating to official assisted immigration - Crown Lands and Immigration Office Consignment/Sub-Series:00001
Official assisted passage passenger lists - use microfiche in the Research Centre.
Unit:7
Official assisted passenger lists, 1855
Surname Year First name/List No Volume
GAHAGAN1855 Margaret 55/19 7
GAHAGAN 1855 Thomas 55/19 7
You would need to visit either the State Library of South Australia or SA Records.
Their dau Ann was not born on the journey to Adelaide. It would have been noted on the Ships List website.
Cando