The 27-year-old Derby defender marked his first start for Ireland with a goal in the 4-0 friendly demolition of Georgia in June, but did not even make Giovanni Trapattoni's provisional squad for Friday night's clash with the Swedes and Tuesday's trip to Vienna.
He told the Derby Telegraph: "I am disappointed not to be in the squad, to be honest. I feel like I have started the season well and when I went away with Ireland last time, I thought I did well.
"But that's football and I can only keep doing what I'm doing. I'm sure if I keep playing well for Derby, I'll get back in there.
"I'd be lying if I said I wasn't disappointed. I think my form over the past three seasons has been consistent enough to be in with a shout.
"But it will give me a chance to recharge my batteries (during the international break) as I've played a lot of football."
Trapattoni has had to contend with a series of problems with defenders since naming a 29-man party, with Sean St Ledger having withdrawn through injury and Stephen Kelly for family reasons, while full-back Marc Wilson has been nursing an ankle problem.
But when the Italian called for a reinforcement, it was Bristol City's Greg Cunningham who got the nod to join up with the squad earlier this week.