Slogan Cancels. 
What is a slogan cancel? It is a cancel that promotes a specific event or one that encourages actions by the reader. A good example of the former is “Buy at the Saorstat Fair, Ball’s Bridge Dublin May 5-9” while latter is to “Post Early for Christmas” promotes a pattern of action to benefit both the mailer and the Post Office. While the first appeared in 1918 “Feed the Guns – Buy War Bonds” during the period of the British General Post Office, they have proliferated on through the GPO period into the An Post era.
Since cancels were not considered a philatelic item (“it’s just a cancel”), consequently they were not saved by collectors until recently. Today the early cancels are quite scarce, and are often priced accordingly.
One of the best references for slogan cancels is “Irish Slogan Postmarks 1918-1994” by Uwe Netzsch. FAI Volume 19, 1995. James MacKay, the noted English philatelic author produced the now out-of-print “Irish Slogan Postmarks”.
An annual summary of slogan cancels is also published in the EPA “Revealer”. A searchable CD ROM of the first 50 years is available from the EPA Special Programs Manager.