The Ulster flanker has been ruled out of next Saturday's clash against South Africa in Dublin and the following week's appointment with Fiji at Thomond Park, Limerick.
Ferris, 27, suffered an ankle injury during Ulster's RaboDirect PRO12 victory over Edinburgh three days ago.
The Irish Rugby Football Union said no decision has yet been made on his availability for Ireland's autumn finale against Argentina on November 25.
Ferris' absence is the latest major injury blow for head coach Declan Kidney as Ireland prepare for their return to action after last summer's 3-0 Test series defeat in New Zealand.
Centre Brian O'Driscoll and hooker Rory Best have already been ruled out of the autumn games because of injuries, while lock Paul O'Connell has only taken a limited part in training.
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