Ulster lock Iain Henderson says that Ireland head coach Joe Schmidt has given his players no indication of who is likely to face Wales on 8 August.
The World Cup warm-up match in Cardiff is the first of four games Ireland will play before the World Cup.
"Joe is so hard to predict when it comes to selecting squads," said Henderson.
"He keeps us on our toes at all times, so it's typical of him to be keeping us in the dark."
Ireland take on Wales at the Millennium Stadium before facing Scotland, Wales again in a return match in Dublin and finally England.
Schmidt has been at pains not to reduce his resources by whittling down his training group before he names his 31-man World Cup squad on the 31 August deadline.
|Ireland's Rugby World Cup warm-up games|
|Wales (away)||Saturday, 8 August|
|Scotland (home)||Saturday, 15 August|
|Wales (home)||Saturday, 29 August|
|England (away)||Saturday, 5 September|
"It's making everyone work harder and harder all the time, and that in turn probably puts more pressure on him just to make those selection decisions," added Henderson.
"We're not even sure what the squad will be like for next weekend's game yet: Joe has kept us in the dark on that and no doubt that's deliberate."
Ireland are better-placed than ever to reach their first World Cup semi-final in the tournament, hosted by England, which starts on 18 September.
Schmidt's men will open their World Cup Pool D campaign by facing Canada at Cardiff's Millennium Stadium on Saturday, September 19, before clashes against Romania, Italy and France.
Highly-regarded New Zealander Schmidt has just signed a contract extension until June 2017 and Ireland approach the World Cup as double Six Nations champions.
Schmidt could reasonably be expected to mix up selection at the start of the warm-up fixtures, but Henderson said that Ireland's players fully expect to be left guessing on selection until at least the early stages of next week.
The Ireland squad will return to their Carton House camp in Kildare on Sunday night after a week with their provinces.
"Joe wouldn't be doing anything if he didn't have a logical explanation for it," said Henderson.
"If you were to take the time and ask him why we were doing specific things then he would always have an answer for that. He's meticulous in his planning - everything we do is for a reason.
"Again, that's just another way of everyone staying sharp and on their toes.
"It builds that competition again - there's absolutely no chance of anyone thinking they can have an easy week expecting not to play next weekend or anything like that.
"In a way it allows you to focus completely on what is happening day by day."