Leinster were on the front foot from the restart with the ball worked patiently across for Lowe to score once again in the 43rd minute.
Northampton responded with their most consistent spell of possession but were undone by timely interventions, first from Joe McCarthy and then from Andrew Porter.
This fixture has already featured one famous comeback, with Leinster having come from 22-6 behind to win the 2011 final 33-22, and hopes of a similar turnaround were sparked by Northampton forcing a pair of scores late on.
In the 58th minute, when Leinster lost a line-out in their own half, Hendy took advantage of the broken play to kick ahead and gather to give his side a lifeline.
Still 10 points ahead, Leinster looked to respond quickly only for Byrne’s penalty to drift wide. Croke Park’s partisan crowd incongruously cheered as the ball sailed off line, greeting the sight of Ireland head coach Andy Farrell on the big screens.
Northampton would bring the game within a score when Tom Seabrook went over in the 74th minute, doing so after the Saints had fought back to win a scrum penalty.
Smith’s conversion cut Leinster’s lead to just three and the Saints threated one of the great comebacks in the final minutes only for replacement back-rower Jack Conan’s turnover with 40 seconds remaining to bring one last Croke Park roar.
Still top of the Premiership, Northampton will take plenty from their final quarter back into domestic action.
Leinster, meanwhile, will prepare for their third Champions Cup final in a row.
Leinster: Frawley; Larmour, Henshaw, Osborne, Lowe; R Byrne, Gibson-Park; Porter, Sheehan, Furlong; Molony, McCarthy; Baird, Van der Flier, Doris (capt).
Replacements: Kelleher, Healy, Ala’alatoa, Jenkins, Conan, McGrath, H Byrne, O’Brien.
Northampton: Furbank; Ramm, Freeman, Dingwall, Hendy; Smith, Mitchell; Waller, Langdon, Davison; Moon, Coles; Lawes (capt), Graham, Augustus.
Replacements: Matavesi, Iyogun, Millar-Mills, Mayanavanua, Scott-Young, James, Litchfield, Seabrook.
Referee: Mathieu Raynal (Fra)