The success or failure of ancestral research depends, not only on the sources available but also on the initial information supplied by you.
So it is important that you find out all the information you can about your ancestors at the outset. Start with your own family, old letters, certificates of births, deaths or marriages, oral tradition, family Bible, unusual family names etc. You should also consult documentary evidence in your own country, such as census returns, church records, land records, naturalisation papers, military records, passenger lists, passport records, obituaries, tombstones etc. A place of birth or a townland is useful as it can help narrow down the possibilities, especially when it relates to a parish where a surname commonly exists.