The 25-year-old flanker hasn't played since aggrevating a knee problem during Ulster's Heineken Cup win over Aironi at the end of January.
It had initially been hoped that Ferris would be in contention for an international recall for the games against Wales and England on March 12 and 19 respectively but Ireland team manager Paul McNaughton has explained that this is no longer the case.
"I think it's fair to say that we can rule Stephen Ferris out of the rest of the Six Nations so he is gone for those two games," McNaughton told The Irish Times.
Ulster will hope the latest assessment of Ferris' injury doesn't impact too heavily on what is gearing up to be a crucial end-of-season run in.
The Northern Irish province currently sit just outside the Magners League Play-Off places with a game in hand and will soon be stepping up preparations for a Heineken Cup quarter-final against Northampton on April 10.
"He is down for a consultant's visit to London this weekend and they will know more about that, but it is too early to write him out of the Heineken Cup game at this stage," added McNaughton.
Meanwhile, scrum-half Tomas O'Leary, who was selected alongside Ferris for the Lions' last tour before injury prevented him from travelling to South Africa, could yet feature against Wales and England.
O'Leary missed the recent win over Scotland at Murrayfield with a back problem but Ireland remain hopeful he could return for Rounds 4 and 5.
"Tomás O'Leary is continuing rehab with his back and we are not ruling him out of any games at this stage," explained McNaughton.