Unable to break into the first team at St Andrews, he was loaned to League One Oldham Athletic on November 18, 2004 to gain first-team experience.
He made a bright start to his spell at the Latics, scoring twice in his second game, away at Bradford City.
He played his part in Oldham's season, taking the free kick which led to the winning goal against top-flight Manchester City in the FA Cup, and ending up by winning the club's Players' Young Player of the Year award.
At the start of the 2005-06 season he re-joined the Birmingham first team squad.
On September 20, 2005, he made his Blues first-team debut in the Carling League Cup away at Scunthorpe United, coming on as a substitute in the 26th minute to replace Muzzy Izzet and receiving praise from manager Steve Bruce for his display.
Only days later, on September 24, 2005, he made his full home debut against Liverpool in the Premier League.
He was sent off in the 83rd minute for handling the ball on the line, an incident which handed Liverpool a penalty and a 2-2 draw.
Apart from the disappointment of his dismissal, he went on to receive credit from fellow team mates Mikael Forssell and Mario Melchiot for his overall performance.
On July 30, 2007 he was again loaned to Oldham, the loan period to run at least until January 1, 2008 but the move was blocked following the collapse of Birmingham's purchase of Hossam Ghaly.However the loan was finally confirmed on August 11, 2007.
When his loan at Oldham expired, he returned to Birmingham City. On January 4, 2008 he signed for Leeds United on an emergency loan, with a view to ensuring his availability for the next day's game prior to completing a permanent transfer the following week.
He made his debut in the 3-0 win over Northampton Town, putting in an impressive performance to get the man of the match award.
Kilkenny signed a three-and-a-half year deal with Leeds on January 7, 2008 for a fee of £150k.
Even though Kilkenny was born in England his family had moved to Australia when he was young and he grew up in Brisbane.
He was eligible to play internationally for Australia, England and Ireland. Shortly before the 2006 FIFA World Cup he committed to play for Australia.
He was among a group of young players, selected by manager Guus Hiddink, who trained with the Australian squad in Holland and Germany prior to the tournament.
Kilkenny made his international debut for Australia on June 7, 2006, coming on as a late substitute in their 3-1 victory in a World Cup warm-up game against Liechtenstein in Ulm, Germany.
In 2007 he played a vital role with the Australian Olyroos in their 2008 Beijing Olympics qualifying campaign.
Kilkenny admitted it was a tough decision to reject Oldham's contract offer and sign for Leeds.
A fee had been agreed with Birmingham and Kilkenny would have become the highest-paid player at Boundary Park, but the lure of Leeds was too much to resist.
He made a total of 59 appearances with Oldham, scoring 8 goals. Kilkenny also turned down Doncaster Rovers.
Kilkenny said: "It has never been a secret that I love Oldham, I love the people and I love the club, so leaving wasn't easy. It was a very tough decision, but everyone knows Leeds are a massive club and I had to take that into my mind.
"I had a couple of offers and there were one or two other clubs sniffing round, but it's hard saying no to Leeds and I hope the Oldham fans don't hold it against me.
"I loved being there and I had a good relationship with the crowd, so hopefully they will appreciate what I did. I tried my hardest in every game, including for the last couple of weeks when I knew there was a possibility I wouldn't be going back.
"I wanted to go out on a high and leave them in good stead, which is what happened because we were winning lots of games."
Kilkenny's final Athletic appearance was on New Year's Day when Leeds found their unbeaten home record being shattered in a 3-1 defeat. He had a superb game and is in no doubt his new boss, Dennis Wise, was impressed by what he saw.
Kilkenny said: "I'm sure it was part of the reason he signed me, although I did okay against Leeds earlier in the season and he did know me a bit already.
"I bumped into him in Australia during the summer, we discussed what was happening at Birmingham and he said he would keep an eye on it. Dennis Wise was a quality midfield player, so I'm looking forward to learning from him, plus all the other coaching staff at Leeds.
"I think it helps when your manager understands you, and that was definitely the case with John Sheridan and Oldham.
"Shez helped me to improve as a player because I had a great relationship with him and I knew he trusted me out on the pitch.
"He gave me the confidence to try things and express myself, which a certain other manager at Birmingham (Steve Bruce) never did.
"I've left Oldham with good memories and I was really happy when I saw their brilliant FA Cup result at Everton ( a 1-0 win at Goodison Park.)
"At the start of the season I could have signed for one or two clubs in the Championship, but I signed for Oldham and I hope that shows how much I wanted to play for them."