Chris Henry suffered "a temporary blockage of a small blood vessel in his brain" on the morning of Ireland's clash with the Springboks, team bosses have confirmed.
Ireland medics initially diagnosed a virus on the morning of the November 8 clash before suspecting a severe migraine, but specialists have since determined the Ulster flanker suffered a more unusual condition.
The 30-year-old missed Ireland's 29-15 Springboks victory and now "needs further investigation and specialist opinion," according to an Irish Rugby Football Union (IRFU) statement.
"Chris Henry suffered some weakness on the morning of the South Africa game and a viral illness was suspected, he recovered quickly and the initial diagnosis was of severe migraine," read the IRFU statement.
"Further tests however have shown he suffered a temporary blockage of a small blood vessel in his brain.
"He is at home now and is well but needs further investigation and specialist opinion."
Henry impressed as Ireland's regular openside flanker through last term's Six Nations-winning run, replacing the long-term injured Leinster back-rower Sean O'Brien.
With O'Brien again sidelined this term, the dogged Ulsterman was in line to continue his blossoming loose-forward partnership with Jamie Heaslip and Peter O'Mahony.
Rhys Ruddock claimed his first Ireland try against the Springboks, stepping in for Henry at the last minute two weeks ago.
The Leinster flanker, son of former Wales international Mike Ruddock, could now come back into contention to face Australia in Dublin on Saturday.
Ireland's injury update bulletin also confirmed Mike McCarthy suffered a concussion in Sunday's 49-7 victory over Georgia in Dublin, and the Leinster lock will now undergo the return to play protocols.
Centre Jared Payne will test out the progress on his foot sprain by running under observation on Monday afternoon - further evidence the Ulster back remains in contention to face the Wallabies.
Hooker Rory Best is now back to full fitness after calf trouble that sidelined him from the Springboks match, and the experienced front-rower could well now start from the off against Australia this weekend.