Town & County Collecting. 
Many people of Irish descent have a particular feeling or draw to the town, county or counties from where their ancestors came. This can make an interesting, and challenging yet very personal collection. The more populated counties, such as Co. Dublin have many opportunities due to the larger volume of mail, but also have a challenge in finding examples from the smaller post offices that have been long closed or consolidated. More rural counties, such as Donegal, offer challenges in both areas. To increase the challenge factor try to find cancels from the time period when your ancestors lived there!
Since a collection is what a person chooses to make of it, there is no restriction on putting in ephemera, that is, related documents and even photographs.
The Eolai an Phoist, the handbook of the Irish Post Office, is available in several editions, and contains a listing of post offices and postal agencies by county. Back issues are occasionally seen in on-line auctions.
For those seeking Co. Clare information, “Introduction to the Post Offices of Co. Clare” by Mackey & Cassiday is available from the EPA Special Projects Manager. Back issues of “The Revealer” are available on a searchable CD ROM and may contain what you are seeking. For example, the Winter 1996 issue contains an article on the “Postmarks of County Westmeath 1839-1860″. If not, write an article on your collection for The Revealer.