QSqlDatabase::port is ignored by the IBASE driver

At least with FireBird, the port number for the server can be specified in the database connection string, separated from the server name by a slash:

dbserver/3051:\DB\MyDb.fdb

alternatively, use the service name:

dbserver/gds_db:\DB\MyDb.fdb

http://www.destructor.de/firebird/multiinstance.htm

Interbase is using a portmapping, the default name is "gds_db".

For interbase it seems to be possible to apply a different mapping in the connection string <hostname>/<mapping name>. See articles:

http://dn.codegear.com/article/33616
http://dn.codegear.com/article/25843

Since Interbase 2007 it is possible to use a portnumber instead of the mappingname, see releasenotes:

http://support.borland.com/servlet/KbServlet/download/5336-102-658/ReleaseNotes.pdf (# 228356)