The 22-year old is set to claim his 17th cap for Ireland U21s when Noel King's side line up against Germany in Heidenhaim on Tuesday evening, with kick-off at 5.15pm (6.15pm local time).
Cullen has been an ever-present for the side since his debut against Qatar in 2015 and the Charlton Athletic loanee has set his sights on hard work and a senior call-up in the future.
"It's going to be a bit emotional, no doubt about it. It'll be the final time playing with the Under 21s," Cullen admitted.
"I think I can only use that emotion to drive me on in my career and aim towards that senior team.
"I've got to get my head down and keep improving with each week, and after that, try and catch the eye of Martin O'Neill. I've come a long way and international football is huge in my development but now I've got to look to make the next step.
"It's been a huge honour to my captain my country. It's the dream of every young player to captain their country at any level so to do it at Under 21s level is fantastic."
Cullen currently plays for Charlton Athletic in the English League One, having signed a loan deal in the summer transfer window from his parent club West Ham United.
With his grandparents hailing from Leitrim, Cullen says lining up for Ireland has been the proudest moments in his career.
"My grandparents are from Leitrim. Unfortunately, they've passed away now but every time I go out on that field for Ireland, I do it for my family but especially in memory of my grandparents," Cullen said.
"It's been the proudest moments of my career to play for Ireland and it'll be the last time we play together as a group so we want to represent ourselves well for the final time."
Ireland suffered a 6-0 defeat in the same fixture at Tallaght Stadium but the U21 skipper is determined not to see that happen again.
"There are so many different styles of teams at international level and there really isn't any such thing as an easy game. Every game, you've got to try and figure out the opposition and adjust so it's been a massive learning curve," Cullen continued.
"It's going to be tough but the group are determined to end the campaign in the right way so we'll be looking to get a result."
Shaun Donnellan arrived in Germany on Saturday after serving his one-game suspension for the match against Israel. Noel King has a fully fit squad to choose from ahead of the game in Heidenhaim, Germany.
Republic of Ireland U21 squad v Germany
Goalkeepers: Kieran O'Hara (Manchester United, loan Macclesfield Town), Liam Bossin (Nottingham Forest).
Defenders: Danny Kane (Cork City, loan AFC Flyde), Corey Whelan (Liverpool, loan Crewe Alexandra), Ryan Sweeney (Stoke City, loan Mansfield Town), Ryan Delaney (Rochdale), Jimmy Dunne (Burnley, loan Hearts), Sean McLaughlin (Cork City), Shaun Donnellan (Yeovil Town).
Midfielders: Josh Cullen (West Ham United, loan Charlton Athletic), Harry Charsley (Everton), Liam Kinsella (Walsall), Jordan Shipley (Coventry City), Jamie McGrath (Dundalk).
Attackers: Reece Grego-Cox (Crawley Town), Ronan Hale (Birmingham City, loan Derry City), Joe Quigley (Maidstone United), Jake Mulraney (Hearts).
UEFA U21 European Championships Qualifying - Group Five Fixtures
16/10 - Germany v Republic of Ireland, 5.15pm (6.15pm local time), Albstadion, Heidenheim.