The Ulster player suffered some weakness on the morning of his side's 29-15 victory over South Africa earlier this month, forcing him to withdraw from the starting line-up.
Early suspicions were that the 30-year-old was suffering from a viral illness, but Henry recovered quickly and was given the initial diagnosis of severe migraine.
However, further tests have shown he suffered a temporary blockage of a small blood vessel in his brain.
A statement from the Irish Rugby Football Union read: "He is at home now and is well but needs further investigation and specialist opinion."
Mike McCarthy is undergoing the return to play protocols after suffering concussion in the 49-7 win against Georgia on Sunday, while Jared Payne (foot) is set to run under supervision on Monday and Rory Best (calf) has returned to full training.
Ireland, who beat Georgia 49-7 on Sunday, host Australia on Saturday.