A greeting for Ireland's dispersed sons and daughters ;-)
The movement, migration, or scattering of a people away from an established or ancestral homeland
Dia is Muire Dhuit
"God and Mary be with you" the correct response to "Dia dhuit" - "May God be with you", which is the way people greet each other in Irish.
So I suppose this shirt means: "May the scattered people be with you", but really it just sounds funny :-)