He was a son of the famous Nest, daughter of Rhys ap Tewdwr, the last king of Deheubarth (South Wales).

On returning to Wales, Fitz-Stephen helped him to organise a mercenary army of Norman and Welsh soldiers, including Richard de Clare, 2nd Earl of Pembroke, alias Strongbow.

The main invasion party landed near Bannow strand, County Wexford with a force of 30 knights, 60 men-at-arms and 300 archers.

Dublin was besieged by a large army under the command of the King of Ireland, Ruadhrí Ua Conchobhair.

Fitzstephen sent his best troops to assist the besieged garrison in Dublin, however this left Wexford vulnerable to attack.

However, after the Norman garrison in Dublin managed to break the siege, the Irish took Fitzstephen and his men prisoner and retreated, burning Wexford as they withdrew.

[4] Taken prisoner by the MacCarthy Reagh in 1171, he was by then surrendered to Henry II of England, who appointed him lieutenant of the Justiciar of Ireland, Hugh de Lacy.

Robert rendered good service in the troubles of 1173 and was rewarded in 1177 by receiving from the king of England, jointly with Miles de Cogan, a grant of the kingdom of Cork, "from Lismore to the sea".

Having no living male heirs, Fitz-Stephen eventually ceded these territories to Philip de Barry, his half-nephew around 1180:"Robert FitzStephen to all his lords, friends, and dependents, French, English, Welsh, and Irish, greeting.

A compromise agreement was reached that allowed the barons to hold seven cantreds near Cork with the remaining twenty-four being retained by the native princes.