The north coast Seahawks have lost just once this season and boss Gary Taylor is upbeat for the fifth-round clash.
"We're top in our division and through to a cup final, so we will go there in good spirits," said Taylor.
"We've worked on a plan, on how to set up and we've nothing to lose, so hopefully we can spring a surprise."
Taylor had spells with Coleraine and Linfield as a striker while there's plenty of Irish Cup experience at the Mullaghacall club.
Assistant manager Anthony Gilmore scored for a losing Carrick Rangers side in the 1995 final while defender John Neill played for Coleraine in the decider 13 years later.
Glenavon will have to keep an eye on winger Ryan Doherty in the Mourneview Park tie.
"I can't understand why Ryan is still with us - he's been superb and we think he can play at a higher level," added Taylor, who has a 21-year association with Portstewart.
"Indeed, we have three or four here who could play at a higher level and the test for them is to prove they can do it on Saturday.
"To get the holders, in the first game of their defence, really is a great game for us.
"We will not go out all guns blazing as we respect Glenavon and they have quality players.
"Portadown scored two late goals to beat us in the Irish Cup a couple of years ago and I hope we give a good account of ourselves again."
There's live coverage of the Irish Cup fifth round on BBC Radio Ulster and the BBC Sport website
Intermediate minnows Portstewart are relishing the opportunity to rock local football by knocking out Irish Cup holders Glenavon on Saturday.