She won in a record time of 28 minutes 41 seconds in the 8 kilometre race at the Sport Ireland Campus.
McCormack's sister Una Britton took bronze in the same race - they both next head to Lisbon for the European Championships.
This comes after she secured her place at the Toyko Olympics in 2020
McCormack ran a new personal best time in the Chicago Marathon.
Fionnuala came fifth overall in a time of 2 hours 26 minutes and 47 seconds, knocking four minutes of her previous personal best, to go 2nd in the all-time list of Irish marathon runners, behind Catriona McKeirnan.
Suspended Bray GP Before The High Court Tomorrow
Young Wicklow Readers Celebrated at Ballywaltrim Library
Met Éireann Issues Status Yellow Rain Warning for Wicklow
Tender Issued For DART Charging Infrastructure In Wicklow Station
Almost 1,200 Wicklow Children Waiting for Early Years Places
