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Coventry-born Harness moved from Burton Albion in the summer for an undisclosed fee and will be watched again by McCarthy’s scouts in the coming weeks.
The 23-year-old came up through the Burton academy and scored six goals in 23 games last season, mostly from a right wing position.
Pompey boss Kenny Jackett commented on Harness’s versatility at his unveiling and remarked that the player is at home on either wing or at wing-back.
“Terry Connor and myself both saw Marcus play with Burton last season and we will have him watched again when the season kicks-off next weekend and see how he gets on,” said McCarthy ahead of an appearance at the FAI Festival of Football in Meath on Thursday.
“He has told us he wants to play for Ireland and we will follow through on that now and help him sort the paperwork from his side.”
Set to name his squad early next month for the Euro 2020 qualifier against Switzerland and the friendly with Bulgaria, McCarthy has kept a close eye on his players’ movements over the summer.
The Ireland boss was delighted to see Troy Parrott start for Spurs against Juventus on Sunday and is taking more than a passing interest in James McCarthy’s future at Everton.
“We know all about Troy and Robbie Keane has been keeping in contact with Spurs about his fitness and his progress,” added McCarthy as his side go into the new season on top of the Group D table.
“He was very unlucky with injuries at the start of the year and was very disappointed to miss the Under-17 European Championships with Colin O’Brien’s side.
“But he is back now and the fact that Spurs kept him out of Tom Mohan’s squad for the Under-19 Euros indicated to me that they were going to use him in pre-season.
“Troy got a start on Sunday against a top Italian side which was great for him. He is young and we have to remember that but this is good news for the player and for Ireland.”
With Crystal Palace keen to sign James McCarthy and Danny Crowley set to make his Championship bow for Birmingham at Brentford on August 3rd, McCarthy will have more midfield options for the Swiss game.
He added: “I’ve chatted with James and he knows my view that he needs games on a regular basis. He is a hugely important player for us but I need to see him play week in and week out now that he is fit again and he understands that.
“Danny’s move has been monitored as well. I did go to see him in the Dutch Cup final when he only came on as a late substitute but it is a bit easier to get to Birmingham matches! He is one of the players we will observe closely when the season kicks-off.”
With one eye on Southampton, as Michael Obafemi and Shane Long return to action after injury lay-offs, McCarthy is also monitoring Luca Connell’s progress since his move to Celtic.
And he will be in Dundalk on Wednesday night for their Champions League clash with Qarabag.
“It’s been a great week for Irish football,” stated McCarthy. “Tom Mohan’s team have been brilliant out in Armenia at the European Under-19 Championships and I wish them well against Portugal on Wednesday.
“I’m going to be in Dundalk that night as they continue to fly the flag in the Champions League and we will have someone at the Shamrock Rovers game against Apollon Limasol on Thursday night as well after their impressive win against Brann.”
A keen golfer and golf fan, McCarthy was captivated by Shane Lowry’s Open win at Royal Portrush on Sunday.
“I know Shane loves his sport and is a keen Ireland fan,” said McCarthy. “He gave everyone in Ireland something to shout about and be proud of at Portrush on Sunday and I am sure I speak for everyone involved with Irish football when I congratulate him on his win.”