Horgan has enjoyed an outstanding season, scoring nine tries in 26 appearances for Leinster, and started the Heineken Cup final against Northampton.
The 32-year-old had given himself an outside chance of participating at the World Cup, only to see his prospects ruined by a knee injury that required surgery.
Alongside Ulster duo Ian Humphreys and Nevin Spence (both shoulder), the Lions veteran has been excluded from the 43-man squad named for the first training camp of the summer.
An Irish Rugby Football Union statement read: "The only players unavailable for selection are Shane Horgan, Ian Humphreys and Nevin Spence, who all had to undergo surgery on long-standing injuries.
"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."
Horgan's inability to feature in any of the warm-up games against Scotland, France (home and away), Connacht and England effectively ends any lingering ambition of forcing his way into coach Declan Kidney's plans.
Leinster's Shane Jennings and Tony Buckley of Sale, however, remain in the mix.
The IRFU statement continued: "The squad includes several players who are in the process of returning from long-term injuries to begin their pre-season after lengthy layoffs.
"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."
Felix Jones, Mike McCarthy, Conor Murray and Brett Wilkinson are the only uncapped players among the 43 to be involved in 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."
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).
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.