Milltown were cruising to victory, leading by 10 points inside the final 10 minutes, before their Tipperary opponents cut the deficit to just two points by full-time.
Goals from Emma Buckley and Roisin Howard had Cahir dreaming of an incredible fightback and when Aisling McCarthy pointed with just 15 seconds remaining, Milltown were clinging on at Parnell Park in Dublin.
But the clock was against Cahir, who were also beaten finalists in 2012, as Milltown held on to claim a very first club title for Westmeath in any of the adult grades.
Ultimately, it was a stunning start to the second half that paved the way to glory for Milltown.
They were a point down at the break, 0-6 to 0-7, but opened with a blistering run of 1-2 without reply.
County star Kearney rattled home a brilliant goal 27 seconds after the restart and when Sinéad Boyce and the excellent Leanne Slevin added points, Milltown were four points clear, 1-8 to 0-7.
Milltown went on to dominate the second half as Cahir struggled to gain a foothold again and when 14-year-old Sarah Dillon (daughter of manager Tom) came off the bench to fire home a superb goal with eight minutes remaining, Milltown were 2-14 to 0-10 clear.
But Cahir had a kick or two left in them and those late strikes from Buckley and Howard had Milltown sweating.
R Howard 1-3 (0-1f), E Buckley 1-0, A Moloney & A McCarthy (1f) 0-3, A Corcoran 0-2, O McEniry 0-1.
R Kearney 1-3, L Slevin 0-5 (3f), S Dillon 1-0, S Boyce 0-2, R Dillon, B Beglan, F Kearney & L Keena 0-1 each.
N Murphy; O Casey, S O’Donnell, L McEniry; A.M. Ryan, E Buckley, E Flannery; A.M. O’Gorman, A Moloney; A Corcoran, A McCarthy, C Casey; K Downey, O McEniry, R Howard.
A O’Donnell for Flannery (48), R O’Donnell for S O’Donnell (48), G Quirke for O McEniry (53), K Arbuckle for Corcoran (57).
G Byrne; Louise Kiernan, H Walsh, R Halligan; K Boyce-Jordan, J Rogers, R Dillon; R Kearney, T Dillon; B Beglan, F Kearney, L Slevin; Laura Kiernan, S Boyce, L Keena.
S Dillon for Beglan (48), K Walsh for Keena (54).
G Carmody (Mayo).