The certificates are not intended as proof of Irish citizenship. Instead, they are certificates of Irish heritage.
The exact size of the market for a heritage certificate is not known. But it is anticipated that many descendants of Irish emigrants would wish to buy one to display in their homes or as gifts for their children.
You can read more in an article by Paul Cullen published in the Irish Times web site at http://www.irishtimes.com/newspaper/frontpage/2010/0621/1224272953828.html