A try by Anne Marie Donovan saw the two props linking, with a superb offload by Sue Hughes on the outside to set up the score on 15 minutes.
Navan breached the St. Mary's defensive line on two occasions in the first half, but Gill Lalor covered back magnificently to prevent the tries.
A penalty by Sue Hughes on the half hour mark from 45 metres out gave the hosts an 8-0 lead to take into the break.
In the second half, St. Mary's took a tighter hold of the game; controlling possession and territory.
Ten minutes into the half, Una O'Murchu went over in the corner after some slick passing across the back-line.
Two further tries followed in rapid succession as the St. Mary's backs began to create space.
Tries by Gillian Walsh (58 mins) and Paula Fitzpatrick (60 mins) showed the slickness and attacking authority of the St. Mary's backs.
Not to be out done, the forwards rumbled over for a try of their own by Sue Howley in the 68th minute.
Having won a lineout 10 metres out from the Navan line, they secured possession and powered over the line with a well-controlled maul.
Navan, to their credit, pushed hard in the closing minutes for a consolation score but could not breach the St. Mary's defence.
With top flight champions UL Bohemians' match away to Clonmel being postponed, Blackrock took advantage as their second win of the league run - a thumping 45-0 triumph over Galwegians - saw them move four points clear at the top of the table.
Highfield are in second position following an impressive 42-0 win over Ballincollig at Woodleigh Park.
The Women's All-Ireland League will crank up another gear next weekend, with the opening fixtures in Division 2 North and Division 2 South as well as round 3 in Division 1.
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