from an online dictionary:

Dago:

Interestingly, much like Guinea, this derogatory term for Italians did not originally refer to Italians. Dago comes from the Spanish given name Diego. It nautical in origin and originally referred to Spanish or Portuguese sailors on English or American ships. This usage dates to the 1830s. The meaning eventually broadened to include anyone from southern Europe, before narrowing again and restricting usage to Italians. The sense meaning Italian dates to at least the 1870s.

from the same site:

Wop:

Similar to wog, wop is often thought to be an acronym for With Out Passport, supposedly used on Ellis Island to designate immigrants without proper papers. This pejorative term for an Italian was probably first used in America, but its roots are in the Romance languages.

Like many other etymologies contained in these pages, this one is not certain, although most authorities agree on the likely origin. It probably derives from the Italian dialectal guappo, or thug. This in turn derives from the Spanish guapo, meaning a dashing braggart or bully, and which eventually derives from the Latin vappa, meaning flat wine or scoundrel. A related word is the French wape, meaning rogue.

The earliest usage in the OED2 dates to 1912 and is spelled wap, which supports the derivation from guappo. The next usage cite, from 1914, uses the more familiar spelling of wop.

Wolfgang