Nick McCarthy is back at Leinster - having left the province for Munster in 2019.
The scrum-half, 26, is a product of the Leinster academy and played 36 times for the province before moving to Munster ahead of the 2019-20 season.
His Blues debut came against Toulon in the Champions Cup in 2015 and he last played for Leinster in the 2019 PRO14 final win over Glasgow.
His move to Munster didn't bring the more regular game time he was looking for - the emergence of Craig Casey behind Conor Murray meant that McCarthy only made four Reds appearances last term.
"I want to thank everyone for my time in Munster Rugby." McCarthy said.
"It’s a special club to play for with a great rugby culture.
"I have really enjoyed the experience, learned loads and made friends for life.
"I am excited now to get back to Dublin in the summer.
"I will continue to challenge myself, compete at the highest level and hopefully add value to the squad in Leinster."
Leinster boss Leo Cullen said: "Nick is a player we know very well.
"We know the qualities that he brings as a player but also as a person and the additional experience he now has from two years in another environment is an added advantage.
"We look forward to him joining us for pre-season and welcoming him back to UCD."
