You should check for all variants of the surname when you carry out your research. This link will show you the variants. It also shows the percentage of McGrees in the country (taken during Griffith's Valuation in the mid-19th century)http://www.irishtimes.com/ancestor/surname/index.cfm?fuseaction=Go.&UserID=. As you can see, a massive 92.3% were in Co. Kilkenny. So in some ways this is both a good and bad thing. Good in that most of the McGrees lived in the right area for you. But bad, as it may take lots of time to go through. It would help a lots, if you knew a more precise location.
Did they get married in Ireland or Australia?
Do you know when or under what circumstances they left Ireland?
Have you checked the immigration records in Australia? You have the name of the ship and a date, so this will help a lot. Generally, more information was given at the port of arrival rather than the port of departure. The University of Woolongong has produced, on microfiche, a complete index and transcript of all information concerning immigrants of Irish origin recorded on ships' passenger lists between 1848 and 1867. These are useul for finding out an exact place of origin as well as parents' names. The Public Record Office of Victoria has good online databases of settlers at www.prov.vic.gov.au Otherwise, other records may be found in the Colonial Office Papers of the UK National Archives, class reference CO 201. This class contains a wide variety of records, including petitions for assisted passages, emigrants' lists, records of emigrants on board ship, peitions from settlers for financial assistance and much more.
Civil registration records are available from the General Register Office (GRO). These start from 1864 however. You can access the website here: http://www.groireland.ie/research.htm
You could also try checking the land records called the Tithe Applotment Books (1823-38) or the later Griffith's Valuation (1848-64). Griffith's is freely available here: www.askaboutireland.com or here: www.failteromhat.com Failte Romhat has lots of other useful links you could try looking at.
We would love to hear from you if you do come to Ireland in July. Hopefully, some of the McGrees in Co. Kilkenny will see this message and make contact with you. Before you arrive, perhaps you could also write a letter/email to the local newspapers in Co. Kilkenny. It might be worth trying!
Please make sure you link anyone else in your family who is interested in their Irish heritage to our site - and indeed anyone else you know of Irish heritage.
Genealogist (Ireland XO)