According to the US Census, in 2003 34 million Americans claimed Irish Ancestry. This is almost 12% of the US population and almost 10 times the number of people living in Ireland in 2003 (3.9 million).
This is listed in the in the Census "Facts for Features" web site section, which is described as:
"Facts for Features and Special Editions consist of collections of statistics from the Census Bureau's demographic and economic subject areas intended to commemorate anniversaries or observances or to provide background information for topics in the news."
For example, I found out that the week of September 16th is "Unmarried and Single Americans Week" and that there are 92 million Americans 18 and older who are in this group.
While I can't say any of this is overly relevant to the topic of this blog, we are trying to provide pointers to data sources and "Facts for Features" is a data source:).