The manager has stuck with the same group which overcame the Netherlands
Two goals from Millwall's Aiden O'Brien and another strike from Wolves' Anthony Forde helped the Irish to victory that day.
King said: "Portugal are another top tier nation so this will be another huge challenge for us but after our last two victories over Holland and Italy we will go into the game with a lot of confidence.
"Obviously any time you play a team like Portugal you expect it to be a difficult game but we've gone with the same squad as last month as they all did well.
"We are still in the getting to know you stage with some of the newer faces and these matches and training sessions will be very useful in preparing for the Uefa Championships qualifiers which begin when we play the Faroe Islands in August."
Ireland U21 take on their Portuguese counterparts at the Tallaght Stadium at 19:45 GMT on Monday, March 25.
Republic of Ireland U21 squad: Aaron McCarey - Crawley Town FC (on loan from Wolves), Sean McDermott - Sandnes Ulf, Shane Duffy - Everton, Anthony O'Connor - Burton Albion (on loan from Blackburn Rovers), Derrick Williams - Aston Villa, Matt Doherty - Wolves, Joe Shaughnessy - Aberdeen, Carl McHugh - Bradford City, Cian Bolger - Leicester City, Samir Carruthers - Aston Villa, Callum Reilly - Birmingham City, John O'Sullivan - Blackburn Rovers, Kane Ferdinand - Peterborough United, Frankie Sutherland - Portsmouth (on loan from QPR) Chris Forrester - St Patrick's Athletic, Sean Murray - Watford, Anthony Forde - Wolves, Aiden O'Brien - Millwall