Ireland has four provincial boundaries: Ulster to the north, Leinster to the east, Munster to the southwest and Connaught to the west. The provinces date back to before the 11th century, before the Normans invaded Ireland, when the land was governed by kingship, clans and wars (Duffy 1999; Ouimette 2005). The four major clans that correspond to the provinces are O'Niell (Ulster), MacMurrough (Leinster), O'Brien (Munster) and O'Connor (Connaught) (Ouimette 2005: 23).
Irish counties can be thought of as equivalent to a state in the United States (Ouimette 2005: 23), though they took nearly twice as long to create them all and are based upon prominent family lines. The counties within each province are as follows:
Ulster - Donegal, Derry (Londonderry), Antrim, Down, Tyrone, Fermanagh, Monaghan, Armagh, Cavan.
Leinster - Longford, Westmeath, Meath, Louth, Dublin, Kildare, Offaly (Kings), Leix (Queens), Wicklow, Carlow, Wexford, Kilkenny.
Munster - Clare, Tipperary, Limerick, Kerry, Cork, Waterford.
Connaught - Mayo, Sligo, Leitrim, Roscommon, Galway.
For a better view of Irish maps and more, check out this website.
Note: Northern Ireland consists of Ulster province minus county Donegal.
Ireland is also divided up into Dioceses, Parishes (civil, Church of Ireland, Catholic), Baronies, Poor Law Union areas, Electoral Division and Townland (Ouilette 2005). These will become more important when the research can continue in Ireland. Rather than bore you (and to get back to my other projects), I shall leave these further descriptions until I encounter them in the records.
Duffy, Sean. Part I: Origins in Atlas of Irish History, ed. Sean Duffy. 1999. Gill and Macmillan, Dublin.Gardener, David; Harland, Derek and Frank Smith. Genealogical Atlas of Ireland. 1964. Desert Book Company, Salt Lake City, Utah. Ouimette, David S. Finding Your Irish Ancestors: A Beginner's Guide. 2005. Ancestry, Provo, Utah.