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As you can see from the link below, Lusk is a Civil Parish in Co. Dublin:
Records for the corresponding Roman Catholic Parishes of Lusk begin in 1757 and Rush in 1785, though there are some gaps in the series. There are some places where you can search online for the marriage of Ellen and Thomas, however these websites do not have complete collections and many do not cover the period after 1900:
www.familysearch.org has a huge database of genealogical records including some church records for Ireland
http://churchrecords.irishgenealogy.ie/churchrecords/ have begun a project to upload Irish church records to their site free of charge. However, so far only Counties Dublin, Kerry, Carlow and parts of Cork have been covered
As the couple married prior to 1864 there will be no state/civil record of their marriage. Other important sources include the Tithe Applotment Books and Griffith?s Valuation. Even though these sources only record the head of the household, the information is very important and can be valuable:
Tithe Applotment Books (1823-1837): http://titheapplotmentbooks.nationalarchives.ie/search/tab/index.jsp
Griffith?s Valuation (1847-1864): http://www.askaboutireland.ie/griffith-valuation/
The National Archives of Ireland: http://www.census.nationalarchives.ie/search/ you can search the 1901/1911 Census on the National Archives page too, for instances of the family name at that time
Remember to post any new information that you find here. The more information you post, the more likely it is that one of our volunteers will be able to advise or assist you. Also include information concerning which sources you may have already used so others may further your search.
Best of luck with your search