You will need to track back through US Census and passenger ships records. You will need a parish in Westmeath where you can locate baptism records.
The Tithe's a tax for the Church of Ireland is now used as
a census substitute as most government documents were destroyed in 1922 Four Courts Fire and due also poor conservancy. The Tithe's took place between 1820-1839
Westmeath: 1827 there are 213 head of householdswww.genealogy.nationalarchives.ie
The next census substitute is 1854 in Westmeath:
There are 329 Fagan's.
There is a parish record database with an Irish History Center that has records for Westmeath. You can search without a parish but as you can see there a lot of Fagan's.
You can search with just a name and a birth year and get
a record with father and mother's names, mother's maiden
name, place/parish and date.
Ireland is a difficult place to search at best.www.familysearch.org
has RC Parish records, scroll down main page to UK/Ireland, click, next page down to Ireland (7) and then search for names. Ancestry also has the collection and the original connection is at the National Library in Dublin.
There are 49 Fagan's in the Westmeath telephone book in a variety of communities.