Replication is a task that can mushroom out of control very easily, so I
hesitate to suggest this.  But if you just want to propagate changes to one
table in one direction (from Ingres to PostgreSQL) then maybe it won't be
too bad.

One fairly simple approach would be to set up an "after" rule on the Ingres
table in question, to invoke a procedure that inserts into a queue table,
with an appropriate discriminator so that you know if it's an
insert/update/delete.  You could then write an application that opens a
connection to both DBMSs to propagate changes from the Ingres queue table to
the PostgreSQL target.  You could perhaps use an Ingres database event to
signal the the program to check the queue.  You can add all kinds of logic
to do robust hand-shaking and error recovery too.

Alternatively you can Google for any of the various replication
products/project out there. Maybe one of them will fit.

Roy