He left the Irish Province in the Spring of 2011 to join Lazio Roma, before moving to Zebre at the start of the 2012/13 season.
To date he has played 20 RaboDirect PRO12 games for the Italian franchise and has made six appearances for Zebre in the Heineken Cup.
Speaking about moving to Ravenhill, Ryan said:
"I would like to thank Fabio Ongaro, the coaching staff and Zebre organisation for the opportunity they have given me. I have develope d a lot as a player and a person during my time in Italy. I believe that Zebre are progressing and definitely a squad whose arrow is pointing upwards."
"While a part of me is sad to leave, the opportunity to join a team with the quality and character of Ulster was too great to pass up. I'm excited to start a new chapter in my career with a team I have always admired for their work ethic, style of play and physicality."
"I would like to thank David Humphreys and Mark Anscombe for the opportunity. I look forward to working hard and being part of a team and organisation who compete at the highest level of European Rugby."
Ulster's Director of Rugby, David Humphreys, added: "Dave is an experienced Irish prop with solid experience in both the PRO12 and Europe. He adds further squad strength to the key position of tighthead prop and I have no doubt that he will be a huge success at Ravenhill."
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