You are correct, the Irish company must be audited.
If the company is part of a group then you must look to Section 280B CA 2014 for the group overall, Section 280A CA 2014 has no relevance therefore with regard to audit exemption. Section 359 CA deals with audit exemption for groups.
Section 359(1)(b) defines a group company, as referring to that company, together with all its associated undertakings, and for the purposes of this paragraph undertakings are associated if one is the subsidiary undertaking of the other or both are subsidiary undertakings of a third undertaking. It does not have geographical boundaries.
Section 359 then refers you to Section 280B to assess if the group overall is considered a small group. Therefore in your case assuming the group does not meet the requirements of Section 280B CA then no entity in the group can claim audit exemption and your accounts must be audited.
In terms of the new subsidiary which filed its annual return late Section 364(2) Companies Act 2014 states that if any of the company’s within the group miss the annual return date then no group company can claim audit exemption. Section 61 of Companies (Accounting) Act 2017 inserted Section 364(4) which states where the first annual return following incorporation is filed late for any group company (i.e. first annual return date is 6 months from the date of incorporation) audit exemption can continue to be availed of.
Therefore if it was the subsidiaries first annual return, the the group audit exemption can still apply, however where it was not the first annual return, all group companies must be audited.