Ruddock had been looking to return to Ireland since leaving Sixways to see more of his rugby-playing sons Rhys and Ciaran.
Flanker Rhys appeared in both the Six Nations and the IRB Junior World Championship before being called up to make his senior debut in Ireland's match against Australia.
And Ruddock senior has vowed to build on the foundations laid by his predecessor.
He said: "The importance of this age grade level is vital and the chance to work in Irish Rugby again was one I did not want to pass up on.
"Allen has made some real strides with the team, with their championship success last season and runners up spot the previous year a testament to this.
"I am looking forward to working very closely with him to ensure there is continuity with what I want to bring to the team and most importantly keeping the Under-20 team as the focal point for the identification pathway for future professional players.
"The focus with any underage team is trying to develop players and also trying to add a winning culture to the ones that pass through the team each year.
"A good part of my coaching career has been spent here in Ireland and with my own personal experiences through Ciaran and Rhys playing for the under 20's, I feel I have a good understanding of the systems and I am really looking forward to getting involved to help develop young players."