Irwin has been allowed to return home after suffering a ruptured bicep in the game against Batley last month.
Verlinden, who agreed to join the club in the autumn last year, has been waiting for visa clearance for several weeks.
Coach Andy Hay said: “It’s a big relief to finally see Ryan here at the Big Fella’s Stadium and our stadium name certainly matches Ryan’s physique!
“It’s been a long await for Ryan, coaching staff, officials and supporters and we’ve all had to show a lot of patience.
“He’s really excited to be here and even after that long flight from Australia he was showing a lot of enthusiasm when meeting his new colleagues. He hasn’t been here 24 ours yet and he really wants to integrate and is already on the look out to find work in the UK.
“He’s a big guy and will really add another dimension to us, you can tell he’s going to be a big asset on and off the field and I’m sure his arrival has been worth the wait and all the hard work that has gone in behind the scenes.”
Verlinden is likely to make his Rovers bow in the Challenge Cup game against Catalans on Sunday evening, and will be in contention to make his Championship debut against Leigh at the Summer Bash next weekend.
On Irwin, Hay added: “Sam ruptured his bicep in our victory against Batley Bulldogs which is quite a serious injury. It’s fair to say also that over the last month in which Sam hasn’t been able to play or train he’s been suffering from a bit of home sickness so whilst he recovers, this is an ideal opportunity for him to return home and catch up with friends and family. The door is open for his return when the time is right.”
Irwin, 22, has made 10 appearances for Featherstone since arriving from Burleigh Bears.