The pair said “I do” with a “very informal” Catholic blessing at 3pm thanks to a close family friend.
This weekend had been the day they were due to marry after postponing their nuptials planned for August 1st last year due to Covid-19 restrictions.
The Lord Mayor, also the first female to marry in office, is due to finish her year-long term as city Lord Mayor on June 28th.
She was given away by her mother Stella and the couple’s three-and-a-half year-old daughter Alex.
The bride and groom, 41, arrived together to a private room as they have been living in an apartment above the Mansion House.
Ms Chu, 40, said: “We decided to move the wedding forward due to Patrick’s father Peter being unwell, and we wanted peace of mind for our parents and family.
“There are 12 of us altogether celebrating, and we will have a larger event on June 4th next year in Poulaphuca House, Co Wicklow. We were both nervous with excitement as was Alex leading up to today.”
Wearing an ivory lace and crystal encrusted dress bought in Ciara Bridal in Powerscourt Town Centre two weeks ago and hot pink pumps, Patrick wearing a blue suit and Alex in an ivory and gold sparkle dress with red high top boots, the couple became husband and wife following a 15-minute ceremony.
The Lord Mayor added: “It’s a very, very special day for us, and it was important to us to share it with the public as the 300-year-old building is owned by the people.”
Ms Chu, a barrister, pointed out that without their respective parents, her mother Stella, Patrick’s parents Peter and Mary along with her brother Joe and sister-in-law Sang, they would not have been able to carry out their political duties over the past year.
“Patrick and I have been together 15 years, been through so many highs and lows, three local elections and one general election, me being seriously ill 12 years ago, that today really does mean so much to us.”
The couple and their close family will be having some of their favourite food including steak in their private rooms and privately paid for.
The newly-weds plan to have a few days away in Strand Hill in Galway in the next few months as both are keen surfers.
The new Mrs Costello added: “I would really like to thank the public for all their hundreds of kind messages and to those who congratulated us outside the Mansion House.
“I’ve to thank Adonis flowers for my colourful bridal bouquet of peonies and roses and Alison in Blow and Dry hairdressers on Merrion Row who insisted on helping me with my hair today.”
Despite saying she was taking the weekend off she insisted on attending an ant-racism events at Smithfield Square on Saturday as it issue is close to her heart.
She revealed her mother Stella had been subjected to racist abuse on several occasions when she first arrived here from Hong Kong several years ago.
Deputy Green Party leader Catherine Martin, along with hundreds of well wishers congratulated Ms Chu on her wedding. She said: “Best wishes to you and Patrick.”
It is not the first time a Lord Mayor married while being the Capital’s first citizen.
Fellow Green Party colleague John Gormley also married while holding the same office and in the Mansion House.
Former Fianna Fáil councillor, Royston Brady married his now wife Donegal native Michelle McConalogue in December 2003. They now live in Florida.