The actress, who plays the hilarious no-nonsense principal Sister Michael, said she was inspired to take part by the campaign to relax abortion laws in Northern Ireland.
At present a termination is only permitted here if a woman’s life is at risk or if there is a risk of permanent and serious damage to her mental or physical health.
The 1967 Abortion Act has not been extended across the Irish Sea, and a series of groups are calling on the Government to legislate at Westminster to bring Northern Ireland into line with Britain.
Currently starring in season two of the Channel 4 comedy, Ms McSweeney said: “I’m proud to stand beside London-Irish Abortion Rights Campaign and lend my voice in the hope that in the near future there will be no need to fight for basic and humane healthcare.
“I’ve always thought the spirit of Ireland has been female and on this St Patrick’s Day, seeing Irish women support all mna (women), is truly a day to celebrate being Irish.”
Also joining the march will be The Fall and Hollyoaks actress Bronagh Waugh, Labour peer Lord Dubs and Labour MPs Stella Creasy and Diana Johnson.
This year will mark the third time the London-Irish Abortion Rights Campaign has joined the capital’s St Patrick’s Day parade, which attracts around 100,000 spectators.
The group’s float has been designed by campaigner Anna Kealy and will celebrate the Giant’s Causeway.
Cara Sanquest from the group explained: “In the past we’ve celebrated important women from across the island of Ireland. This year we’ve chosen to honour one of our most well-known landmarks.
“For us the Giant’s Causeway’s interlocking columns represent the inter-sectional supporters of the pro-choice community – people from all walks of life in Northern Ireland who come together to recognise the right to abortion care. It also represents a tangible connection between Northern Ireland and the rest of the UK.”
She added: “Our members will be carrying signs of interlocking hexagons in a variety of colours to show the diversity of our community.”The procession will start tomorrow at noon from Piccadilly and will pass some of London’s most famous landmarks.
Ms Sanquest added: “This is a family-friendly event and we encourage as many people as possible to come along.
“It’s a chance for us to show our solidarity with the 28 women forced to leave Northern Ireland every week for abortion care.
“But above all it gives us a chance to celebrate our culture and heritage and drum up support to make Northern Ireland a fairer and more equal place for all.”
In a landmark referendum last May the Republic voted overwhelmingly to overturn the country’s abortion ban by 66.4% to 33.6%.
The eighth amendment, which grants an equal right to life to the mother and unborn, was removed.