The final hours of the January window proved to be frantic at Parkhead with the Hoops also recruiting Diomansy Kamara, Edson Braafheid and youngster Paul Slane.
But it was the signing of Keane which captured the imagination the most and Smith said: "There have been a lot of changes and Celtic have to be congratulated for making the changes there.
"They have brought in some good players and that will be a benefit to them.
"It's obviously raised the interest in Scottish football again, with a player like Robbie Keane coming here.
"That's a good thing for our football overall - although I'm not too sure if I'm that keen on it being good for Celtic."
Smith himself was no stranger to high-profile signings during his first spell at Rangers, with the likes of Paul Gascoigne and Brian Laudrup gracing Ibrox during the 'Nine in a Row' era.
But the Rangers boss admits he thought the days of 'box-office signings' north of the border were long gone.
"I did as a matter of fact," he said.
"The wage levels of players in England and in Europe are making it very difficult for Rangers and Celtic to get the better level of player in.
"I thought that was gone now.
"It may be the way forward that players of the level of Robbie Keane are going to come in on high wages but just on loan.
"But it's good to see in one respect. For Scottish football it has been a bit of a lift but hopefully it doesn't give Celtic too much of a lift."
Smith had hoped the departures of Jerome Rothen and Pedro Mendes would allow him to strengthen his own squad.
But, in stark contrast to Celtic, the midnight deadline passed without any new arrivals for the Scottish champions.
Even so, Smith insisted: "There is no envy. When I took over at Rangers at first we were in decent circumstances and Celtic's circumstances at that time weren't great.
"So you just have to appreciate that situation and, when you find yourself on the other side of it, you have to keep going.
"The players who are here, I can't speak highly enough of them. They have been fantastic.
"We have had a few transfer windows now without getting any new players in and some of them have been working in situations where they have not been sure where their situation is going to lie.
"But every one of them have been committed to the club and that's been terrific. They have been a great bunch of boys to work with.
"Myself and the players face a big challenge between now and the end of the season to try and retain the title.
"There is a little bit of frustration there that maybe we leave ourselves open a little bit to injury and suspension with not being able to add to our group.
"But, all in all, I'm still delighted to be working with the boys who are here."
While Tony Mowbray has been able to bolster his squad throughout January, Rangers came through the weekend action with a 10-point lead at the top of the Scottish Premier League intact.
Asked who is better placed to win the championship, Smith smiled: "We will know at the end of the season, won't we?
"It's never an easy one to answer.
"What we know is that we will have to show the consistency that we have done.
"We have only lost one game this season so I feel we will have to show a real level of consistency over the remaining games that we have left."
Celtic 4/11, Draw 10/3, Kilmarnock 15/2