The hosts goaled within 60 seconds when full-forward Hughie Gahan got the final touch to a long ball from midfielder Darragh Foley.
Louth fought back but were hit by a second goal for the Barrowsiders from midfielder Seán Gannon in the 27th minute as Carlow held a 2-3 to 0-6 interval lead.
A third Carlow goal three minutes after the restart put O’Brien’s men into a 3-3 to 0-6 lead.
Louth’s Seán McCann goaled in the 62nd minute but it was too late.
Scorers for Carlow: S Gannon 1-1, H Gahan 1-0, P Broderick 0-3, 0-1f, K Toner 1-0 o.g., J Murphy 0-2, D O’Brien, D Bambrick 0-1 each.
Scorers for Louth: D O’Connor, R Holdcroft 0-3 each, S McCann 1-0, D Byrne, R Moore 0-2 each, R Burns, T Durnin 0-1 each.
CARLOW: R Molloy; D Kelly, S Redmond, K Nolan; M Fitzgerald, D St Ledger, B Kavanagh; S Gannon, D Foley; E Finnegan, J Murphy, D Bambrick; E McCormack, H Gahan, D O’Brien.
Subs: P Broderick for McCormack (h/t); C Mullins for O’Brien (h/t); E Rth for Foley (42); A Kelly for Finnegan (46); D Moran for Fitzgerald (46); J Kennedy for Mullins (59).
LOUTH: C Lynch; K Toner, D Crilly, D O’Hanlon; A Williams, P Rath, D Finnegan; D Byrne, R Brodigan; B Duffy, D O’Connor, T Durnin; R Burns, H McGinn, R Holdcroft.
Subs: R Moore for McGinn (24); P Nixon for Broddigan (30); D McComish for Finnegan (43); S McNann for Durniln (50); R Quigly for Holdcrost (55).
Referee: E Kinsella (Laois).