The 6ft 7in second row has been named in the invitational side's line-up for their Test against Scotland at Aberdeen FC's Pittodrie Stadium (kick-off 7.30pm, live Sky Sports 1), have missed out on wearing the famous black-and-white hooped jersey last year.
Longwell, who is due at a farewell engagement in Belfast tomorrow, said: "The Scotland Test a wonderful opportunity for me. I was selected for the Barbarians tour to Portugal last June, but unfortunately I was unable to travel.
"I don't want any song or dance made about stepping down. I just want to go quietly.
"Of course, there is the old saying that never say never. I dare say that if a good mate came looking for me to help him out, I may get the boots out again," he told BBC Sport.
"But now is the time for me to think seriously about the next step in my career."
Longwell's long-time provincial colleague David Humphreys, already pencilled in to skipper Ireland against Japan next month, will captain the Bob Dwyer-coached Baa-Baas against the Scots.
In all, there will be seven Ireland players involved, with Girvan Dempsey, Kevin Maggs, Frank Sheahan and Eric Miller also starting, and recent IRUPA Player of the Year Johnny O'Connor set for bench duty.
As well as lining out against Scotland, Dwyer's charges are also primed to face England at Twickenham (kick-off 3.00pm, live Sky Sport 3), next Saturday.
Barbarians president Micky Steele-Bodger said of the Scottish trip: "We have put together a very interesting squad for this year's British tour. There is a lovely mix of home grown talent and overseas players.
"Visiting Aberdeen for the match against Scotland is a new departure but we are confident that it will be a popular one.
"Great touring sides such as the All Blacks, Springboks and Wallabies have played in Aberdeen in the days of the long tours and we are delighted to be part of a new era of top-class representative rugby in the north of Scotland."
SCOTLAND: Chris Paterson (Edinburgh); Rory Lamont (Glasgow), Marcus Di Rollo (Edinburgh), Andrew Henderson (Glasgow), Sean Lamont (Glasgow); Gordon Ross (Leeds Tykes), Mike Blair (Edinburgh); Allan Jacobsen (Edinburgh), Scott Lawson (Glasgow), Bruce Douglas (The Borders), Stuart Grimes (Newcastle Falcons), Scott Murray (Edinburgh), Kelly Brown (The Borders), Allister Hogg (Edinburgh), Joe Petrie (Glasgow) (Capt).
Replacements: Dougie Hall (Edinburgh), Euan Murray (Glasgow), Craig Hamilton (Newcastle Falcons), Andrew Wilson (Glasgow), Graeme Beveridge (Glasgow), Dan Parks (Glasgow), Hugo Southwell (Edinburgh).
(15) Girvan Dempsey (Leinster/Ireland)(14) Brian Lima (Samoa)(13) Matt Burke (Australia)(12) Kevin Maggs (Ulster/Ireland)(11) Sireli Bobo (Fiji)
(16) Raphael Ibanez (France)(17) Cobus Visagie (South Africa)(18) Selborne Boome (South Africa)(19) Johnny O'Connor (Wasps/Ireland)(20) Mark Robinson (New Zealand)(21) Thomas Castaignede (France)(22) Kenny Logan (Scotland)
Referee: Nigel Owens (Wales)