Johnston, 28, rose to fame last year by winning the Spanish Open and has since come to be regarded as one of the biggest crowd-pleasers in the game.
Pieters, meanwhile, made a huge impression on his Ryder Cup debut at Hazeltine last year.
Poulter has become one of the greats of the modern Ryder Cup era.
Tournament host Rory McIlroy will return home to defend the title he claimed in style last year, while world number two Hideki Matsuyama will make his regular European Tour debut having finished joint runner-up at last week's US Open.
"This is certainly one of the strongest fields I've ever seen for the Irish Open, if not the strongest. It's really shaping up for an incredible week," said four-time major winner McIlroy.
"Everybody knows how special this tournament is to me, and to return to Northern Ireland as defending champion, with my Foundation again hosting the event, is a real honour.
"The Northern Irish fans are renowned the world over for their great support. The last two tournaments here in 2012 and 2015 were both sell-outs and the atmosphere was something else.
"I expect it to be the same this time with a field of this depth and such a great venue at Portstewart."
Andrew 'Beef' Johnston has confirmed he will play in next month's Irish Open at Portstewart, as will fellow Englishman Ian Poulter and Belgian Thomas Pieters.