The 23-year-old, who joined the Black Cats from League of Ireland side Derry City last September, has scored five goals in 21 appearances in his first season in the top flight.
McClean earned his first Irish cap as a late substitute in a 1-1 draw with the Czech Republic in March and will be joined by his Sunderland team-mates Keiren Westwood and John O’Shea.
The Derry native paid credit to O’Neill for placing his faith in the inexperienced winger after the former Aston Villa manager replaced Steve Bruce at the Stadium of Light in December.
“He has been tremendous to me,” McClean told the Daily Mail.
“He presented me with my opportunity to play in the first team and I think I took full advantage of that. He’s kept me on the ‘straight and narrow’ since then and has helped me improve as a footballer.
“I try to work hard on my game week in and week out in order to improve and I think that has worked for me. Martin O’Neill’s appointment has certainly boosted my confidence levels and I can’t thank the man enough.”
McClean described his call-up to the Ireland squad for this summer’s European championship as the perfect ending to a “fabulous” season for the Sunderland winger.
“Obviously I was delighted to get the call,” McClean added.
“It’s yet another dream come true for me but this selection caps what has been an absolutely fantastic year for me.
“It really has been an amazing rollercoaster ride for me. To have joined what I consider a top Premiership club and make my debut for Sunderland was absolutely fantastic.
“To then make by international debut was also very special and to have been voted Sunderland’s ‘Young Player of the Year’ was yet another great boost for me.
“But to have now been selected onto the Irish squad to play in the European Championships has really capped what has been a fabulous year.”
MORE: Man United latest news
MORE: Arsenal latest news
MORE: Chelsea latest news
MORE: Liverpool latest news