The 24-year-old has Irish links through his grandfather, and is eager to test himself in the northern Hemisphere after holding an Academy contract with the Waratahs for the past three years.
D'Arcy's successful two-week trial in Belfast has netted him a one-year contract with Ulster. He will make his competitive debut for Ulster tonight, donning the number 15 jersey for the visit of reigning Magners League champions, the Ospreys.
Ulster Rugby have also confirmed that Paul Emerick will be joining the province on a short-term contract (three months) as a direct replacement for the injured Andrew Trimble.
Trimble sustained a finger injury whilst on tour with Ireland in the summer and is not expected to return to rugby until the middle of October at the earliest.
With important Magners League and Heineken Cup fixtures between now and then, Ulster have recruited experienced international Emerick to provide cover for the outside backs.
Emerick has extensive experience of European rugby having played for both the Newport Gwent Dragons and Italian club side Parma and has been capped for the USA Eagles over 30 times, most recently in the summer's Churchill Cup campaign.
The 30-year-old from Iowa will link up with Ulster next week.