Dallin:
As a surname Dallin is English and means one's ancestors came from the town of Dalling in Norfolk, England. Dalling meang "settlement of the followers of a man named Dalla" in Old English.
http://behindthename.com/bb/arcview.php?id=272113&board=gen
Gannon:
Mag Fhionnáin - (Mac) Gannon - Co Mayo
MacLysaght's book "The Surnames of Ireland" says the following:
(Mac) Gannon Mag Fhionnain (fionn, fair). An old Erris family. The Irish
form is Mag Canann in Co. Clare. IF (Irish Families) Map Mayo. See
MacConnon