The First thing to do is to find out as much as you can about your ancestors in your own country from books, records and documents in libraries and state archives, tombstone inscriptions etc.
Oral evidence is also important.
In most states and provinces, cities and towns, there are genealogical societies which will give you advice about your research. If you visit Ireland to pursue a search on your own you will find the necessary records and documents are spread amongst many different archives. Which has official records or the ninetenlh century, large collections of private papers from the seventeenth century and copies of many pre-1900 church and school registers. The National Archives, Dublin. This includes the Public Record Office of Ireland and the State Paper Office.