Barrett powered home with a thumping header in the 36th minute to give Dundalk the lead before Ronan Finn, with the aid of a Pat Sullivan deflection, put the result beyond the midlanders just four minutes after the restart.
The Lilywhites will now face Cork City or Bray Wanderers in the showpiece at the Aviva Stadium on November 8. And, with the SSE Airtricity League title all but secured, the Lilywhites know that victory in the final on November 8 would complete the club’s third league and FAI Cup double and first since 1988.
Longford goalkeeper Paul Skinner should have been tested as early as the fourth minute. Sean Gannon’s brilliant pass cut the Longford defence apart but Ciaran Kilduff could only steer his effort wide of the post after leaving Pat Sullivan in his wake.
The home side remained on the front foot and Mountney set up Richie Towell for a pop in the 13th minute. Towell shifted the ball onto his left foot but the shot lacked the venom to bother Skinner.
The league’s top scorer went closer four minutes later, curling a free-kick just over Skinner’s crossbar after Philip Gannon was penalised for a late tackle on Mountney, 25-yards from goal.
A sustained period of pressure by the Lilywhites ended with Towell curling another left footed effort past Skinner, only to see it come back off the crossbar.
The goal that divided the sides at the break arrived four minutes later. Daryl Horgan’s corner from the right found Paddy Barrett and the centre back’s header had enough to beat Pat Sullivan on the far post.
Longford’s hopes of finding a route back into the game were dealt a hammer blow just four minutes into the second half. Daryl Horgan picked out Ronan Finn and his drive took a wicked, wicked deflection off the unfortunate Sullivan, leaving Skinner completely flatfooted.
From then on it was a case of how many Dundalk would get with Horgan, Finn, Towell and Dane Massey all seeing efforts just miss the target.
Tony Cousins turned to his substitutes bench in a bid to salvage something but David O’Sullivan’s 82nd minute shot into Rogers’ midriff and a late Gary Shaw drive were their only chances of note.
DUNDALK: Rogers; Gannon, Boyle, Barrett, Massey; Towell, O’Donnell; Mountney (Meenan 42), Finn (Shields 75), Horgan; Kilduff (McMillan 81).
LONGFORD TOWN: Skinner; Haverty, Sullivan, Gorman, Powell; Rice, Gannon; Shannon (Deady 86), Salmon (O’Sullivan 60), Simon (Mulhall 66); Shaw.
REFEREE: Paul McLaughlin.