Former Sunderland man Dennehy was one of a handful of players still under contract at Cork, but last week he agreed a deal with the club which allowed him to leave on a free transfer, and the Kerry native has today agreed a switch to Rovers.
O'Neill said: "He scored seven goals from wide areas last season and I think we are signing one of the best attacking players in the country."
Rovers are also keen on midfielder Turner, but only if he can get out of his move to Sligo Rovers. Former Bohemians man Turner agreed to sign for Sligo at the end of last season, following a spell with Dundalk, but he's now keen to have that deal called off so he can link up with O'Neill at Rovers.
SHELBOURNE have signed Premier Division duo Sean Byrne and Andy Haran.
Defender Byrne made a number of first-team appearances for Bohemians last season but has now been allowed to leave Dalymount Park and Shels boss Dermot Keely moved quickly to sign the 19-year-old. Shels have also snapped up striker Haran on loan from St Pats.
DROGHEDA have signed two more players from St Pats with the arrival of midfielder Gareth O'Connor and striker Glenn Fitzpatrick.
Pats boss Pete Mahon has launched a major clear-out of last season's squad, with 18 players leaving Richmond Park since the end of last season, and Drogheda have snapped up three of them, midfielder O'Connor, forward Fitzpatrick as well as defender Jamie Harris.
Drogheda boss Alan Mathews is still in the hunt for new players and plans a scouting trip to Scotland next week as he tries to land a keeper to challenge former Celtic man Paul Skinner, as well as another centre forward.