Louise Minihan, a member of Éirígí, a militant political party, threw diluted red paint over the minister, covering parts of her neck, chest and hands.
The 29-year-old Dublin City Council representative attacked Ms Harney as the minister was turning the sod on a new mental health unit at Cherry Orchard Hospital in Ballyfermot, west Dublin.
A large number of people with families attending the hospital, as well as local politicians, including Ms Minihan, were invited to the ceremony on the hospital grounds. A small group of around 10 protesters were outside the main gate.
Ms Minihan was arrested by gardaí under the Public Order Act and taken to Ballyfermot Garda station. She was not detained for questioning and no statement was taken from her. Once her identity and address were established she was released.
A file will be sent to the DPP. Gardaí yesterday stressed that the attacker was an invited guest and that they had no role in checking those invited and were there to manage any potential trouble by protestors outside the gate.
The incident sparked public debate with the minister and the councillor going on national radio. Speaking on the Joe Duffy radio show, Ms Minihan, a former Sinn Féin councillor, claimed the attack was “symbolic” and that the red paint represented “blood on the hands” of Ms Harney.
“We are just trying to show the anger that is out there,” she said.
The majority of callers condemned her actions, with two callers supporting them. Earlier, Ms Harney told RTÉ she would not interfere with the Garda investigation.
A stream of callers contacted the programme to criticise her actions. One woman said it was “a disgrace”, adding that the councillor could have protested verbally, adding that the clinical director of the new facility also ended up covered in paint.
Leaders from across the political spectrum condemned the incident.