You poor POOR wee thing 
Your username is not at all accurate pet - you're no criminal, you made an honest mistake that you tried to rectify because you are an honest person - and the reward for your honesty was that they've treated you shockingly badly throughout this. I know that they see shoplifters do the craziest things & use any excuse in the book & very frequently use prams/their children to help them them steal, but they'd no business eating the head off you (to put it mildly...) for what was so clearly an honest mistake! And as others have said, what sort of shoplifter comes back to the shop wanting to put an item back/pay for it & apologise (as opposed to the whole "I've lost my receipt but I want a refund" thing...)?!
I think that as well as pressing to have the CCTV of the incident preserved, it might be worth contacting the shopping centre to see if they have any to prove that the security guard was telling a pack of lies about you going back to the store because you saw him following you. As a PP suggested, trying to get hold of the lady in the other shop to see if she'll confirm your version of events might also be as well.
Consulting a lawyer is absolutely the way to go, whether or not you receive a summons to be cautioned. I've no idea of procedure in UK or in Eire, but if it's possible to have solicitor attend the Garda Station with you if told to attend about a caution, I would.
The response from the store manager was utterly callous & I think that you'd do well to escalate that complaint. Criticising you for being upset in the circumstances is particularly unpleasant & uncalled for - spiteful, even, really. Again, I know shoplifters will turn on the waterworks & have a sob-story (or three) to tell, but there's a difference between those kinds of forced histrionics & someone in genuine distress.
If you've got a sympathetic [line] manager &/or HR person it might be worth letting them know what happened? It's not as if the Garda will be coming up to your workplace & hauling you away if they DO try to caution you, so I'd think it's very much up to you to do what feels comfortable when it comes to telling work - though possibly someone who knows better will be along presently to tell you otherwise!
I know it is not The MN Way, but I do wish I could give you a hug, or at least some snuggles from my cats or something. Because that was a horrible thing to happen to you & the horribleness is getting dragged out a bit.
Am v glad your DH is being supportive & hope that you feel better soon. The moral highground is so much yours you could go full "Sound of Music" opening on it. And if you're Maria von Trapp (& I suppose making dresses from curtains would save you from great lumps of store detectives acting the big man) we all know who the security man & nasty Garda are, right...?
Please be gentle with yourself, you've been through a lot. Best of luck getting it all sorted 