Mr Gannon (55) was babysitting his grandchildren Leila (12) and Sean (6) in his daughter Jade's house in Skerries when he was stabbed to death at the front door last week.
Michael, who was known affectionately as "Mixer" to his friends, was remembered at a moving ceremony in the Church of Annunciation in Finglas led by parish priest Fr Eamonn Cahill.
Earlier in the week his partner of 14 years, Nicola Lee, described Mr Gannon as a "loving grandfather" and a "good man" who loved her "unconditionally".
Fr Cahill offered the family a "sad, sad welcome" and that this time last week such an event would have been the "furthest thing" from their minds.
He also spoke of their tragedy as "unfair, unnatural and simply not right".
A rendition of Amazing Grace was sung from the altar as his remains were carried into the church.
Fr Cahill said that Mr Gannon, a father of four, would "go out of his way to be helpful and caring" to people. And he joked that as a professional roofer would be called upon in heaven and if there were any problems "he'd be on the job".
After the ceremony friends told the Herald that he was hugely popular in the Finglas area and was always smiling.
Mr Gannon's daughter Jade delivered the second reading and family members carried the coffin.
Other tributes at the funeral described the devoted grandfather as "popular," evidenced by the huge numbers in attendance on a bitterly cold morning.
The parish priest said "there was so much more that Michael wanted to give but his book remains unfinished".
"The difference he made in your lives cannot be destroyed by his death," Fr Cahill said.
A spray of white lilies and a framed picture of the Finglas grandfather rested on the coffin. Some of Mr Gannon's favourite songs – including James Taylor's You've Got a Friend – were played during communion.
This is the second tragedy to befall the family. Mr Gannon's nephew Daniel O'Haire passed away last year after spending 11 years confined to a wheelchair after he was stabbed in the neck in 2003.
Mr Gannon's remains were taken to Glasnevin Crematorium after the ceremony in Finglas.
Mr Gannon was remembered at the ceremony by his four children Jade, Michael, Aaron and Kate.