I think this must have been it. I downloaded a different driver, libdevartodbcpostgresql.4.5.1.so, which claimed to support Unicode. I still can't use credentials through iris, but by putting the user ID and pw in the irisodbc.ini file I was able to successfully connect. Since this is just a personal project that's getting shut down in a few months, this works for me.

My resulting irisodbc.ini file looks like this:

[ODBC Data Sources]
HL7Interface = HL7Interface

[HL7Interface]
Driver = /usr/local/devart/odbcpostgresql/libdevartodbcpostgresql.4.5.1.so
Host = localhost
Database = hl7interface
Username = postgres
Password = <RealPW>

For any future people, I ended up using the driver for PostgreSQL found here: https://www.devart.com/odbc/postgresql/