He was shot at point-blank range in Rowlagh Park, Clondalkin, at about 6.20pm.
He is now in a critical condition in Tallaght Hospital, and it is understood medical staff have put him into an induced coma.
His friend and fishing-pal Michael Maguire told the Herald Mr Sullivan suffered horrendous injuries.
"His jaw is smashed, his voice box got hit, his collarbone was shattered and the bullet punctured his lung as well," he said.
"We just can't believe what has happened. Padser is just a family man, who likes the odd smoke, a pint and a bit of fishing," Mr Maguire said.
Mr Sullivan has lived at Rowlagh Park for more than two decades with his wife Bernie. Both are believed to be originally from Ballyfermot.
They have four children, Tara, Sean, Andrew and Patrick.
Last night daughter Tara returned to the house where her father had been shot. She declined to comment on his condition, or on the shooting. "My dad is innocent," she said.
Mr Maguire added: "We often go coarse fishing in Cavan. We're good mates. We worked together in the building trade for years.
"I hope to God he pulls through, he's my best mate, but I've been told he's not too good and the doctors will be putting him in a coma for three days."
Neighbours said the first they knew about the incident was when a number of vehicles came speeding into the estate shortly after 6.30pm on Saturday.
"There were two unmarked cars, two fire brigades, an ambulance and some squad cars," said one man. "I didn't know what was going on at first but then the word came back up the street that Padser had been shot. It's just so hard to believe."