Dear 310brett
Welcome to Ireland Reaching Out!
Have you already searched for your ancestors online? There are some places where you can search but be aware that 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.
www.rootsireland.ie have a large collection of Irish Church records, however you have to pay to use this site.
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.
Most Irish record sources- church records, land records, census records- are based on specific locations. Until you know a specific location within Ireland, preferably a parish or placename, it may be difficult to do anything more with these records. It may also be the case that births and marriages took place at a time that predates the parish records, particularly as in your case, you search is quite early.
Luckily Brett is not a very common name. There are 4 William Brett marriages listed on rootsireland for example for Co. Sligo. As it is a subscription site I am limited in what I can see. It is useful though, if there are a small number of choices and a name that is not very common. In this way you can assess what is relevant without having to guess which of 1000 Murphys are yours if that makes sense.
Have a look and let me know how you get on
Best wishes
Clare Doyle
Genealogy Support