In total 17 former Lions have been selected in Ireland's 43-man preliminary squad.
But Horgan's hopes of returning to New Zealand six years after he was there with the Lions have been dashed after it was revealed he required knee surgery.
Ulster's Ian Humphreys and Nevin Spence also omitted due to shoulder injuries.
"The only players unavailable for selection are Shane Horgan, Ian Humphreys and Nevin Spence, who all had to undergo surgery on long-standing injuries," read an Irish Rugby Football Union statement.
"The rehabilitation period is such that all three players will not be able to take part in the World Cup conditioning schedule to enable them to build up the required fitness levels.
"Also, none of these three players will be available for the games during the Guinness Summer Series in August.
"The squad includes several players who are in the process of returning from long-term injuries to begin their pre-season after lengthy lay-offs.
"Some short-term injury concerns were Shane Jennings, who broke an arm at the end of the season, and Tony Buckley, who had a procedure on a shoulder injury.
"It is expected that both will have healed sufficiently so as not to restrict the beginning of their conditioning programmes."
Ireland head coach Declan Kidney has named four uncapped players in the squad with Felix Jones, Mike McCarthy, Conor Murray and Brett Wilkinson all included ahead of the three training camps staged in June and July.
"This is the first step in preparation for the World Cup and the group called into our first camp shows that there is competition for places right across the board," said Kidney.
"There are players who we have identified as part of the larger World Cup squad who will not be in the first camp and will be based with their provincial and club fitness staff.
"But that may change as we go through June and July, so none of that larger group are necessarily out of contention for the final squad.
"The aim three years ago was to deepen the strength of the squad and there are a number of players that have come through in that time, either playing with their provinces or with the Wolfhounds and the Ireland team.
"There are big challenges ahead for the group with the coming pre-season and then five games in August."
* Bold denotes a former British & Irish Lion
Ireland - Backs: I Boss (Leinster), T Bowe (Ospreys), G D'Arcy (Leinster), G Duffy (Connacht), K Earls (Munster), L Fitzgerald (Leinster), F Jones (Munster), R Kearney (Leinster), F McFadden (Leinster), G Murphy (Leicester), C Murray (Munster), B O'Driscoll (Leinster), R O'Gara (Munster), T O'Leary (Munster), E Reddan (Leinster), J Sexton (Leinster), P Stringer (Munster), A Trimble (Ulster), P Wallace (Ulster).
Forwards: R Best (Ulster), T Buckley (Sale Sharks), T Court (Ulster), S Cronin (Leinster), L Cullen (Leinster), S Ferris (Ulster), J Flannery (Munster), J Hayes (Munster), C Healy (Leinster), J Heaslip (Leinster), M Horan (Munster), S Jennings (Leinster), D Leamy (Munster), M McCarthy (Connacht), K McLaughlin (Leinster), S O'Brien (Leinster), D O'Callaghan (Munster), P O'Connell (Munster), M O'Driscoll (Munster), M Ross (Leinster), D Ryan (Munster), D Varley (Munster), D Wallace (Munster), B Wilkinson (Connacht).