Ireland has never passed the quarterfinal stages at the Cup.
In the Irish Times, Thornley said the World Cup starting in France next week was the most open of all World Cups.
However, he believes the most likely outcome will see either France, New Zealand, Ireland or South Africa win the Cup, but two of the four will likely miss the semifinals.
"This is the best Irish side ever to reach a World Cup, but the weight of history is against them as well as the draw, even if their schedule is probably ideal. The previous best was probably the class of 2015 before they lost arguably their five most influential players for the quarterfinals.
"Similarly this time around, if they can escape the pool of sharks [Ireland is in a pool with South Africa, Scotland, Tonga and Romania], the big question remains whether the bulk of their key personnel – i.e. Johnny Sexton, the back three, Jamison Gibson-Park, Andrew Porter, Dan Sheehan, James Ryan, Caelan Doris – can stay healthy through weekly knockout matches.
"If so, then more than ever before, Ireland can win the World Cup."
Australia's Cup plans have suffered a blow with an injury suffered by experienced prop James Slipper. Coach Eddie Jones confirmed to media that Slipper, who did not play in Australia's last warmup game against France, had a tendon issue in his foot and was 'tough and go' to start in the opening game against Georgia.
Jones said, "We believe we've got that under control and he'll be back on the paddock after Georgia."
Jones also said he had been through a turnaround since stepping into the coaching role of how he wanted the team to play.
"The play was always to free us up from structure. I don't believe you can copy other teams' structures; you've got to come up with your own way of playing, and so, that's been the goal, but initially, we didn't get it right in how we wanted to play. It's been a fascinating experience.
"You go in with a perception of how the team may be and then the players' talents have really shone through and we're just discovering now how we need to operate as a team, how we can play to our strengths."
Former Springboks prop Tendai Mtawarira believes South Africa can emulate the All Blacks by winning consecutive Rugby World Cups and be the first team to win the Webb Ellis Trophy four times.
The 117-Test veteran told a South African website boksquad.co.za, "Playing in a World Cup is unlike any other tournament, and every country puts so much effort into ensuring they peak at the right time every four years.
"Some teams seem to thrive on the pressure of a World Cup, and others don't, but I think the history of the Springboks at these tournaments speaks for itself.
"South Africans love being backed into a corner and to be the wounded animal that can lash out and fight back to show our true character.
"I think this Springbok team will go to France with a point to prove, and although there are some unfortunate injuries, there are guys within the squad who can step up.
"I believe if we can get past the quarterfinal – that will likely be against New Zealand or France – than I think we'll go all the way."