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.
Wildfire Battle Continues in France & Spain with Thousands of Holidays Cut Short
Wicklow Businesses Are Staying Competitive with Skills Training
Joy Unconfined As Wicklow Town's Shrine To Mayo Bears Fruit At Long Last!
Wicklow Households Among 1.7 Million Impacted By Uisce Éireann's Bank Holiday Repair Plan
Early Hours Rescue Brings Three People To Safety
