St Mullins started brightly and led 0-3 to 0-1 with pointed frees from Marty Kavanagh (2) and James Doyle.
But the game changed completely once Clonkill were awarded a 12th minute penalty for a foul on Anthony Price by Kevin Kehoe.
Brendan Murtagh came upfield to hammer to the net.
The Clonkill centre-back went on to record his sides’s opening 2-1, scoring a second goal from a close-range free on 19 minutes.
Niall Dowdall struck for his team’s third goal on 27 minutes as the Lake County men stormed into a 3-4 to 0-7 interval lead.
When Dowdall again found the St Mullins net early in the second half it killed off any hopes of a revival for the home champions.
St Mullins never demonstrated the passion of their shock win over Mount Leinster Rangers in the Carlow fina.
The Lake County champions demonstrated a far greater level of skill and hunger as they coasted into a home provincial semi-final with Wexford’s Oulart-The Ballagh.
B Murtagh 2-2, N Dowdall 2-1, A Price 0-2, 0-1f, A Dowdall 0-2, P Dowdall, C Thompson 0-1 each.
M Kavanagh 0-6, 0-5f, J Doyle 0-3, 0-1f.
G Healy; S Power, C Scally, J Fagan; L Folan, B Murtagh, A McGrath; P Dowdall, E Price; M Heffernan, A Price, P Walsh; A Dowdall, N Dowdall, A Price.
D Egerton for Walsh (h/t); C Thompson (0-1) for P Dowdall (44); N Mitchell for N Dowdall (49); N Folan for Heffernan (56); M Keegan for E Price (56).
K Kehoe; P Doyle, J Doran, G Bennett; D Whelan, P Kehoe, G Coady; J Kavanagh, M Walsh; P Fortune, J Doyle, M Kavanagh; J Walsh, J Murphy, P Kehoe.
P Connor for Fortune (37); J Murphy for P Kehoe (55); A Doyle for Whelan (58).
G McGrath (Dublin).