Numbers for the Clare airport for last year fell to 1.39m – a drop of more than 230,000 from the 1.62m who passed through Shannon in 2011.
The latest passenger numbers is the worst experienced at Shannon Airport since the 1980s and is the fifth successive year of decline at the airport since a 2007 high of 3.6m.
In 2011, the war in Iraq ended with all US military leaving the country. The same year, 247,761 US troops passed through the airport compared to just 101,808 last year.
The drop in US troops (145,953) over the two years accounts for 63pc of last year's overall decline at Shannon Airport.
Plans are afoot to end all military involvement in Afghanis tan by the end of 2014.
An independent and debt-free Shannon Airport Authority came into being last week.
Rose Hynes – the newly appointed SAA chair – said management are concentrating efforts on increasing passenger numbers from the current low to 2.5m within five years.