Ryan moved to Saints' Senior Academy from the Leinster Academy at the start of the 2017/18 season.
He went on to play a part in a successful Wanderers side, scoring a try in the 2018 Premiership Rugby A League final success against Exeter Braves.
But Ryan was only able to make one first-team appearance for Saints, starting against Wasps in a Premiership Rugby Cup clash in November 2018.
The 21-year-old has been hampered by injuries and has only recently returned from an ACL problem.
Now he will look to build a career with the Pirates in the second tier of English rugby.
"With injury to both John (Stevens) and Matt (Bolwell), plus Will (Wilson) returning to Wasps for A league involvement, it was crucial we strengthen our back row stocks," said Cornish Pirates boss Chris Stirling.
"Paddy has just returned from a long-term injury and is extremely excited to be joining the Pirates on an initial short-term contact.
"This can be a win-win for all involved as, arriving highly recommended, he has a possible chance of a longer-term future."