"Shamrock" is the callsign used by F16's from the 56FW of the USAF
However, any flightplans filed for these obviously don't use the ICAO code for Aer Lingus (EIN), just happens that the spoken callsign is the same.
You'll probably find alot of Military callsigns are the same as some Airline callsigns, for example, FAA Sea Harriers use(d) the callsign "Batman" which is also used by a Swiss bizjet company.
"Silver" & "Snoopy" are another two callsigns that are both used by the Military & by Civil companys that are unconnected
FC