Getting your bearings
There are lots of very good sites and places that explain the different land divisions that have existed in Irish history and how they relate to each other ... sort of! I've tried to summarise the major ones on my "Carving up the Landscape" page and you will find some links there to a couple of very good articles elsewhere. However, and I say this as a geographer, it's very difficult trying to describe and explain them all without it turning into a confusing geography lesson! So, here are my genealogical first steps on navigating some of the layers;
When you at at least know which County your ancestors came from, and generally speaking outside of the major towns and cities, start with locating their Townland;
To find baptisms, marriages and burials, find out which Church of Ireland or Roman Catholic Parish the townland was in.
To find marriages (post 1845) and births & deaths (post 1864), find out which Registration District the townland was in.
To look for land records, find out which Barony and District Electoral Division the townland was in.
To look for Poor Law Records, find out which Poor Law Union the townland was in.