There was more good news as winger Gary McCabe has signed a new contract.
Quigley previously had a spell at Rovers, when he spent time there on loan from Millwall in the 2005 season which saw the side get relegated.
But since then has emerged as one of the league's leading strikers, and Croly hopes he can bring his scoring touch back to Tallaght Stadium.
"It's unbelievable to be back at Shamrock Rovers; I started my career in the league here," said Quigley, who follows Jason McGuinness in quitting Sligo to join the Hoops.
Winger O'Connor, 29, is also back at Rovers having lined out for the side from 2008-10. Quigley and O'Connor have joined James Chambers in confirming a move to Rovers but Croly is still in the hunt for new faces, and Rovers have been linked with another former winger Shane Robinson.
Cork sign Furlong
CORK CITY have made another move in the transfer market by signing up highly-rated Wexford Youths forward Danny Furlong.
Furlong was in demand after scoring 13 goals for Wexford last season and City boss Tommy Dunne, who last week signed up Shamrock Rovers striker Daryl Kavanagh, has completed a deal for Furlong.
"I'm delighted to have signed for City and 'm really looking forward to the next stage of my career," says Furlong.
Meanwhile, Derry City could lose two of their FAI Cup-winning side to English clubs. League Two side Fleetwood Town are interested in striker David McDaid, while Nottingham Forest hope to bring winger Stephen McLaughlin on trial this week.