CGT is a "self-assessed" tax, so the taxpayer must file a return if one is considered necessary even if Revenue have not initiated the return. In your case, even though the net liability is likely to be "nil" I would still consider it necessary to file a return ( form CG1 available from Revenue.ie) to show that there is a "nil" liability. The "nil" liability only arises from application of the personal exemption. If that were, say, income tax, the reduction to "nil" of ones income tax liability from applying personal allowances/credits does not remove the obligation to file a return and the same, I believe, goes for CGT.