Looking for some fun and friendly competition? Dublin hosts a selection of Parkruns (free group 5ks) throughout the city. Locations include:
Griffith Park. One lap of the park is ~ 3k, on mostly flat ground. Not a Parkrun yet, but could be added. Thanks to Tara Lindsay for this suggestion!
…and several others further out. You can find the full list of Ireland Parkruns here.