Derry-based construction group Errigal will invest at least €3.75 million to secure the future of troubled fit-out firm Mac Interiors, should it exit examinership successfully, the High Court has been told. Justice Michael Quinn is being asked to approve a scheme of arrangement to save the UK registered specialist building firm which runs its main operation from Dublin. The Revenue Commissioners, which is facing an almost complete wipeout of €13 million in warehoused tax debt, is opposing the rescue plan.  Usually, the collector general, as a preferential creditor, ranks first in priority in insolvency scenarios but the court heard yesterday…