She acted as one of the Dáil advisers to Ireland's first Green Party Teachta Dála (TD), Roger Garland.

In 1997, she called for a boycott of Dunnes Stores when it was reported to be selling clothes produced in a state-run factory in Myanmar.

[4] She is active in People's Movement opposes further European Union integration and campaigned for the rejection of the Treaty of Lisbon in 2008.

She is known for her victory in the Supreme Court in 1995, in which she argued successfully that it was unconstitutional for the Government of Ireland to spend taxpayers money promoting only one side of the argument in referendum campaigns.

[7] She is also well known for her opposition to nuclear power, genetically modified food, and United States-led military action in various parts of the world.

After the election, she declared herself a candidate for the leadership of the Green Party, but the position was won by John Gormley.

[13] In 2016, McKenna campaigned with Green Leaves, an EU critical faction of the Green Party of England and Wales whose members include Baroness Jenny Jones, for the United Kingdom to leave the European Union in the run up to its membership referendum.