Full-back Nelson and centre Farrell have impressed for the province's Under-20s during their pre-season friendlies and head coach Brian McLaughlin has given them a chance to impress on a bigger stage.
The promising Allen played a number of games for the Ulster Ravens last season and has been handed the right wing berth for the trip to Wales.
The three youngsters are joined in the backs by centre Luke Marshall, out-half Paddy Jackson and winger Craig Gilroy, all of whom made their debuts for Ulster last term. Ian Porter will start at scrum half against the Welsh outfit.
In the pack, Lewis Stevenson will captain the side from second row where he is partnered by Neil McComb. Paddy McAllister and Jerry Cronin will scrum down either side of Ballymena hooker John Burns.
The back row has a balanced look about it with new recruit Mike McComish starting at blindside flanker, Willie Faloon occupying the openside role and Robbie Diack at the base of the scrum.
The replacements bench has an equally youthful look. Academy players Stuart Olding, Blane McIlroy, Ali Birch, Conor Gaston and James Simpson are all in reserve.
Front row players Andrew Warwick, Callum Black and Adam Macklin complete the line-up on the bench.
McLaughlin says his side are hoping to put last week's heavy defeat to Bayonne behind them, stating: "The game in France was very disappointing. It showed that we have work to do to get ready for the start of the season. That's why the game against Neath is crucial for us.
"We played Neath last season in the British & Irish Cup and they were a very big and physical side. They are a good team and play a nice brand of rugby.
"From our point of view it's a great match to take us through to the Harlequins game and the start of the new RaboDirect PRO12 season."
The Ulster boss seems determined carry on last season's strategy of giving opportunities to as many young players as possible.
"We are looking again to extend our squad. Quite a few of the young guys have trained with us all summer, so it's very important that we show them that they will get rewarded if they perform.
"There are young guys coming through like Peter Nelson and Chris Farrell, who are getting great reports coming from the U-20 coaches, so it will be interesting to see how they perform."
Stand-in captain Stevenson is looking forward to leading the province on Friday. "It's challenging but it's exciting because they are a really talented bunch of young guys," he said.
"It's up to some of the older players like me and Robbie Diack to show leadership and make sure that all that energy and enthusiasm is channelled in the right way.
"I've been really impressed with everyone the whole way through pre-season. We've got a great work ethic and hopefully it's going to stand us in good stead for the first few games of the season."
ULSTER (v Neath): Peter Nelson' Michael Allen, Christopher Farrell, Luke Marshall, Craig Gilroy; Paddy Jackson, Ian Porter; Paddy McAllister, John Burns, Jerry Cronin, Lewis Stevenson (capt), Neil McComb, Mike McComish, Willie Faloon, Robbie Diack.
Replacements: Andrew Warwick, Callum Black, Adam Macklin, James Simpson, Ali Birch, Blane McIlroy, Stuart Olding, Conor Gaston.