It was the perfect start to the New Year for Britton, who smashed the women's course record by 16 seconds when she finished 13th overall in 16 minutes 39 seconds.
Making a welcome comeback in second place was her club-mate, Deirdre Byrne.
Less than a month ago, Britton was out-sprinted for bronze at the European Cross-Country Champion-ships in Portugal.
She went on to finish second at a top-quality race in Belgium before the Christmas break.
Her training now is aimed at a good run in the World Cross-Country Championships, which take place in Spain at the end of March.
A fighting third place in the Phoenix Park was Lorraine Manning of Raheny Shamrock, with Linda Bradley fourth.
Pushing the pace from the start of the men's race was Tomás Fitzpatrick of Tallaght, who won the Jingle Bells 5k just before Christmas.
Fitzpatrick came home in 14:54, beating Eddie McGinley of North Down by nine seconds, with Niall Robinson, also North Down, in third place.
Finishing fourth and first master over 45 was former Olympian Noel Berkeley, who made a return to racing a few weeks ago at a Cork BHAA race.
Rathfarnham WSAF's Louis McCarthy was fifth and Paul Cowhie of Civil Service sixth, and second over 45.
Mick Byrne of Slí Cualann, who finished just a place behind Fionnuala Britton in 14th place, was taking his first over-50 win.
Just behind was Raheny's Damien Martin third over 45 and Simon Melyer of Donore first over 40.
Melyer was also helping Donore to team victory over Tallaght and Liffey Valley.
In the women's veteran categories, Elaine Caul of Civil Service beat Mairead Murphy from host club Liffey Valley to take the over-40 prize.
Sheelagh Jones of DSD was first over 50, with Navan's Mary Watters a few seconds behind and first over 45.
Raheny finished first of the teams ahead of Sportsworld and DSD.