Payment of the premium forms paid of any contract of insurance, but not does not specifically define the method. A DD is a a separate contract with options for the insurer to cancel etc. These options wouldn't apply to a standing order, and presumably is the reason why Irish Life are refusing to accept it. I assume they will accept full payment by whatever method as it would be nonesensical to refuse this.
I doubt if any insurer would accept a standing order given that they would forego the legal protections/options available to them under the separate DD contract.