There are three Rugby World Cup winners in the form of Pienaar and his South African colleague Francois Steyn, along with England's Iain Balshaw.
The Baa Baas will also boast at least two players with more than 100 Test caps to their name - long-serving Ireland prop Hayes and Australian flanker George Smith.
The current list for the invitational squad is completed by Italy captain Sergio Parisse, who recaptured some of his best form during this year's Six Nations.
The match on June 4 in Cardiff is to mark the 130th anniversary of the Welsh Rugby Union. The Barbarians will be coached by current Cardiff Blues boss David Young.
"We have the spine of a pretty impressive line-up already and there will be plenty more names in the coming weeks which will whet the appetite for what should be a spectacular game of rugby," explained Young.
"It will be interesting for me personally coaching against Wales and against a side with probably many of my own regional squad players in it.
"I'm particularly looking forward to fielding a back row with (Sergio) Parisse and (George) Smith playing together and, knowing who we plan to slot in alongside them, it's going to be a pretty exciting trio.
"John Hayes has been a thorn in the side of many Welsh teams in the past and you can't ask much more than having three World Cup winners to choose from in your back-line."
Both Hayes and Pienaar, who played against each other at Ravenhill back in October, are hoping to help their respective provinces secure silverware before the season ends.
Munster are on course for a Magners League and Amlin Challenge Cup double, while Ulster are challenging for both the Magners League and Heineken Cup titles.
The 37-year-old Hayes is closing in on his 200th Munster cap. He came off the bench to make his 100th appearance in European competition against Brive on Saturday - 99 in the Heineken Cup and one in the Amlin Challenge Cup.
Tickets for the Wales v Barbarians match are available from www.wru.co.uk/tickets. The prices are adults £40/£30, under-18s enclosure £15, and family of four (2 adults, 2 children) £100.