If Irenaeus and other early Fathers believed in the Millennium on earth and taught this doctrine, why is this not taught by the Orthodox Church (since its position is amillenialism).
Yes, it is true that some of the early Fathers were millenialists (St. Justin the Martyr, for example); Orthodox Christians are well aware of this and of the fact that St Justin acknowledges disagreements on this point of doctrine.
However, we do not see these fathers as being anything but witnesses of the faith of the specific churches they served or interacted with. In this case, it seems that the millenarist school was localized in Asia Minor and connected with the teachings of St Papias. This view was certainly not “taught everywhere, always and by all” to paraphrase the criteria of St Vincent. It is well-established that the churches of Egypt and Cappadocia did not universally accept this interpretation, as Eusebius documents.
In the end, this interpretation fell in disfavor for three reasons:
(1) the broader consensus of the churches did not support it
(2) sound hermeneutics of Revelation did not actually support the literal view
(3) great excess were (and continue to be) connected with millenarism which discredited it
The Council of Constantinople did address the fact that the Kingdom of Christ would have no end (Luke 1), to correct the erroneous view that it would last 1000 years.
However, it does not seem that millenarism is – from an historic Orthodox perspective – strictly speaking a condemned heresy (on account of these venerable fathers who taught it) but rather a tolerable private opinion that should not be taught or promoted because of its overall rejection by generations of Orthodox bishops as a viable interpretation of the text. For more information on this point, also read http://www.orthodoxanswers.org/answer/485/
See also the Introduction to Revelation on the Eastern / Greek Orthodox Bible.