McLeish signed Fahey from St Patrick's Athletic on a three-and-a-half-year deal in January and admits the 26-year-old is now one of the first names on his team sheet at St Andrew's.
Fahey played for his country up until Under-20 level but has still to get the call from Giovanni Trapattoni, although one of his lieutenants, Liam Brady, watched him impress against Aston Villa recently.
McLeish said: "Keith came as a central midfielder. All his good work with St Patrick's was in central midfield.
"He played against Hertha Berlin last season in the UEFA Cup and he was tremendous.
"He is really a central midfielder but we converted him into a wide role when he arrived last January and he has never looked back.
"He is one of the first names on the team-sheet because I can trust him. He takes the ball, has a change of pace and has been a superb team player.
"He has an engine, he covers for his team-mates. He is a great all-round player."
Former Scotland boss McLeish added: "Am I surprised he has been overlooked by Ireland? I am a little bit surprised this time. I know it is difficult picking international sides.
"You are trying to find a position for a new guy but you have got a bit of momentum and you have guys already in the squad.
"That can be a stumbling block in itself but Liam Brady phoned me a few weeks ago and I told him Keith was playing versus Aston Villa and he played really well in that game.
"I am sure Liam would have gone back with a good report. He would have the benefit from knowing Keith from his younger days at Arsenal, although he is nothing like the player now he was then."
Fahey was a trainee at Arsenal before making the move to Villa but never made a senior appearance before returning to Ireland.