Barry Aylett was one of the original Shamrocks when the club formed in 1969.

His son, Richie, was part of the Rocks' great sides in the 90s, while Richie's son, Corey, is making a name for himself in the junior ranks.

Fittingly, Shammies will mark their 40th year in Illawarra Rugby Union with a season-ending showdown against traditional rivals Vikings.

While the Rocks have booked a finals berth, Vikings are destined to finish last. But the men in green know better than to take the Blueys lightly.

"You can throw the form guide out the window whenever Shamrocks and Vikings meet," Rocks coach Barry Jones said.

"It's going to be a memorable day with the anniversary, but it's important that we keep the momentum going into the semi-finals.

"I'm sure Vikings would love nothing more than to take the shine off the big celebrations the club has planned."

The Rocks' three-day party kicked off last night and wraps up with more festivities tomorrow.

Shamrocks formed in 1966 before neighbouring club Woonona Surf arrived two years later.

After overcoming initial resistance by delegates from both clubs, the two entities merged in 1969 and became officially known as the Woonona Shamrocks, rejecting the IRU's suggestion that they be called Northern Suburbs.

The Rocks have won the club championship five times and captured 21 premierships in all grades. Nicholson Park served as the home of Shamrocks rugby before the club relocated to nearby Ocean Park in 2004.

Club spokesman and Shamrocks historian Stephen Saywell said the Rocks take immense pride from being known as a "family club" with a strong focus on their junior players.

"One of the most pleasing aspects of our organisation is the way in which we foster our juniors," he said.

"The club does not pay any players, coaching staff or supporters and relies heavily on the volunteer ethic that epitomises our club spirit.

"There is no equal in Illawarra rugby."

Saywell said the club's identity had changed since its inception.

"Forty years ago our club could be rightly accused of being branded as a bunch of loud, foul-mouthed coalminers from the north," he said.