Lost property on airlines seems to be a big black hole, for a bunch of reasons. It's far more likely just lost in the vast amounts of items people leave behind them every day, than be down to dishonesty. I once tried to look through 3 huge boxes of mobiles left at GVA for my own, without success. I've lost a ton of stuff over the last few years, among which 3 mobile phones (1 returned), 1 ipod (not returned), 1 carton duty free cigarettes (returned, bizarely), a jacket (not returned), keys to a hire car (not returned, and amazingly expensive to replace), and 1 passport (returned, thankfully).
The moment my attention switches from my own world, to trying to disembark and get sorted for the next leg, especially on a routine friday flight home after a ****ty day at work, seems to be a killer for me, and I'd forget anything.
If you've tried the Lost Property at Sligo, I'd try the one at Dublin assuming the aircraft went back there. I've had more success there than any other, although, I have to say statistics are not in your favour. They're down in the arrivals hall, and the number's on the Dublin Airport website (under Contact Us).
Good luck.