The Irish produced a gritty performance in a 17-13 defeat of Scotland in Dublin on Saturday to retain the title, Andy Farrell's side emulating that of his predecessor Joe Schmidt in 2014/15.
It was though a second successive display where they failed to fire, a far cry from the sublime 38-17 dismantling of France in the opening match.
Nevertheless, they showed character in bouncing back from the last-gasp 23-22 defeat by England the Saturday before, which ended their hopes of historic back-to-back Grand Slams.
"They were deserved winners," MacNeill told AFP by phone on Sunday.
"They are still a team in development. They have a powerful line out when it functions and fast rucks. However, the backline is not fizzing on all cylinders as yet.
"Jamison Gibson-Park sets a great tempo, Jack Crowley is getting into it and Bundee Aki is a more impactful player.
"But there were not many sharp coherent back moves."
MacNeill, a key member of two Ireland Triple Crown-winning sides in 1982/85, said it was impressive how Farrell had moved the Irish on from their devastating Rugby World Cup quarter-final loss to New Zealand.
However, the 65-year-old said the quality of the opposition the Irish ran rings around in the first three matches had to be taken into consideration.
"One has to put it in context with the opposition they faced: Wales (31-7) were very poor, both France and Italy (36-0) were weak.
"One has got to realise that. There is still a lot to do."
MacNeill, who also played three Tests for the British and Irish Lions on the 1983 tour of New Zealand, puts the slip in standards in the loss to England and the Scotland game down to their state of mind.
"When everyone is telling you you are not going to lose it insulates you and takes the edge off the performance," he said.
"That was a feature of both the matches, there just wasn't an edge to the Irish game compared to the previous games.
"They were caught short. Once you go into a game with that attitude you can't change that mentality."
'Fantastic player'
MacNeill does not believe Farrell stepping away for next year's Six Nations, due to his being head coach of the Lions for the tour of Australia, will be a factor as the Irish have a "good network of coaches".
His main concern is more about who will replace Peter O'Mahony as captain.
The veteran flanker's post-match press conference sounded like a farewell and MacNeill doubts he will be back to add to his five Six Nations titles.
"I think it is the last we will see of him," said MacNeill.
"All the stuff yesterday, the body language suggested that.
"It was important to compromise when Johnny Sexton stepped down as there was a relative lack of leadership in the team except for him.
"That was the right decision as there was no other option, both Leinster's co-captains James Ryan and Garry Ringrose were not even in the Ireland team for different reasons.
"To go out finishing by winning a championship is wonderful for he is not only a fantastic player but person as well."
MacNeill says whilst the captaincy remains open - another potential candidate Caelan Doris he says needs to "get his game back in order" after two anonymous performances - at least the huge hole left at fly-half by Sexton has been filled.
"I cannot remember a bigger focus on one player stepping into another's boots as with Crowley and Sexton," said MacNeill.
"He has come through very well and that is very good.
"No one is talking about his rivals now."
For MacNeill, the future does look bright in spite of his misgivings, although he thinks the players need a "good break" before the two-Test series with world champions South Africa in July.
"There is much to be positive about," he said. "Irish rugby is in good stead.
"Any year you win a championship and come within a whisker of a Grand Slam is a good year, especially after the disappointment of the World Cup."
