Olympia Theatre announced on Monday that Porter - who co-wrote this year's production with Karl Spain - had stood aside from his role in Polly and the Beanstalk following allegations of inappropriate behaviour.
This would have been Al Porter's 13th year starring in the Christmas panto.
Cowan will now play the character of Polly. RTE reports that he is "very happy to be joining the cast of the Olympia Theatre panto. I am a huge panto fan and treading the hallowed boards of The Olympia is a privilege for me".