My flight is from Chicago to Rome via Dublin Airport. Arriving and departing from the same Terminal 2. Layover is only 1 hour at 5:30 a.m. I'm a US permanent resident card holder with a Colombian passport.
Do I need a visa or transit visa?
According to the official website of Irish Immigration Service, very few countries are required to have a transit visa. Colombia (and clearly the US) are not required to have transit visa. Check this Link for the full list (+ other details) http://www.inis.gov.ie/en/INIS/Pages/Transit%20visas