Cross posted with you earlier WildeWoman. From your link: "If it is decided to administer a caution, the member in charge of the Garda station arranges a suitable time and venue for the administering of the caution by the Superintendent. Once the caution is administered, a copy of the completed Adult Caution Referral Form is given to the offender."
"The entire procedure from the detection of the crime to the completion of the administering of the caution is carried out as soon as is possible, that is, in days rather than months."
From which it would seem possible that the OP may receive a letter or visit from the Garda requiring her to attend a specified police station at an appointed time/date at which time the Superintendent will administer the caution providing the OP gives her consent.
If this should occur, given the circumstances I would strongly advise the OP not to to accept a caution and to opt for prosecution in which case she may be charged with theft contrary to Section 4 of the Criminal Justice (Theft and Fraud Offences) Act 2001.
In this event, it will be incumbent on the prosection to prove that the OP "dishonestly appropriated property without the consent of its owner" (Debenhams) "and with the intention of depriving its owner of it" which is clearly not the case as, after discovering the item that had not been included in the tops she had paid for, the OP returned to the store with the intention of paying for it/or giving it to a member of staff as evidenced by the fact that she was "at least 50' into the store" before she was approached by a security guard. NB it would be interesting to view a tape of that encounter*.
The OP's defence will rely on the fact that "a person does not appropriate property without the consent of its owner if the person believes that he or she has the owner's consent, or would have the owner's consent if the owner knew of the appropriation of the property and the circumstances in which it was appropriated" and that this woman of impeccable character has not acted dishonestly.
Furthermore, the circumstances in which the item was appropriated were known to the owner by means of CCTV and at no time was the OP followed from the store nor was she challenged by a member of Debenham's security personnel when she emerged from the store next door.
In short, it would seem that the OP has been penalised for her honesty and it would be a travesty of justice if she were to be criminalised for it.
*If you can remember approximately what time you returned to the store I suggest your dh (and the letter) requests that the relevant tape also be preserved until your solicitor can view it. Needless to say, keep the letter brief - don't rehash the events of last night and confine it to a request for the tape(s) to be preserved.