A strong second-half performance earned Longford a place in the quarter-finals at Pearse Park last night.
Half-time substitute Liam Connerton instigated Longford's opening goal and then scored their second with just 10 minutes to go.
The home side were reduced to 14 men in the 15th minute when Jason Earley was shown a straight red but they remained composed.
Level at the break, Longford stormed in front on the resumption when Larry Moran found the net and a converted free from Rory Connor put four points between the teams.
Laois found a consolation goal when Paul Kingston blasted a 21-metre free to the net, but Longford held out to secure a spot against Dublin.
ScoreRs -- Longford: L Moran 1-3, R Connor 0-4 (4f), L Connerton 1-0, D McGivney, A McElligott, R McNerney 0-1 each. Laois: P Kingston 1-5 (f), E O'Connell 0-3, E Lowry (f), D Cullen, 0-1 each.
Longford -- C Farrell; M Mulligan, C Farrelly, J Gill; D McGivney, A McElligott, J Earley; T Jackson, A Dalton; C Mallon, R McNerney, S Clarke; L Moran, R Connor, L Sullivan. Subs: L Connerton for Jackson (h-t), K Farrell for Mallon (43), G Connell for Sullivan (47), B Walsh for McGivney (54).
Laois -- A O'Reilly; J Bartley, C Dunne, R C Fennell; M Scully, C McEvoy, R Dowling; J Nerney, D Mahon; N Mullins, P Kingston, D O'Reilly; E Lowry, E O'Carroll, D Cullen Subs: M Lyons for D O'Reilly (24), C Hagan for Dowling (35), N Murphy for Fennell (35), J Moore for Lowry (41), N Brennan for Mullins (54).
Ref -- D Fedigan (Louth)