Here is a connection string I used to connect to an Interbase machine. I didn't use DSN though I used full connection details and You need server licence on the database server. I had the lcpi provider installed, there is a free version but I ended up buying the full version from the Russian manufacturers at:
http://www.ibprovider.com/eng/
OleDbConnection oConn = new OleDbConnection("Provider=LCPI.IBProvider;" +
"data source=<my server name>:<past on datbase to gdb file>;" +
"ctype=win1251;user id=sysdba;password=masterkey");
--
Joe