The back-row forward was sent off in the first minute of the second half for colliding with Irish full-back Jamie Osborne as he jumped to collect a high ball.
The young Scots went on to lose the opening match of the Under-20 Six Nations 38-7.
Morris’s suspension means he will miss Scotland’s next three matches in the championship, against England, Italy and France. He will be eligible for the final game, against Wales on July 13.
The player appeared before an independent disciplinary hearing on Tuesday and while he accepted that he had committed an act of foul play, he did not accept that it warranted a red card.
The disciplinary committee disagreed and found that Morris had infringed Law 9.17 - “A player must not tackle, charge, pull, push or grasp an opponent whose feet are off the ground”.
Such an offence carries a four-week suspension but it was reduced to three on the grounds of his “previously clean record, his youth and inexperience, his conduct at the hearing and his spontaneous apology to his opponent”.
The disciplinary committee, which comprised Rhian Williams (Wales, chair), Val Toma (Romania) and Bogdan Zebega (Romania), heard evidence and submissions from Morris and his representatives, including Scotland coach Sean Lineen.
The player has the right of appeal.
Scotland U20s’ next match in the championship, which is being played in its entirety in Cardiff, is against England on Friday.