A nasty situation.
The DPP takes account of certain factors based on the evidence gathered and submitted by the Gardaí.
Will the prosecution be able to produce the requisite level of proof i.e. proof beyond reasonable doubt ?
Overall, is there a reasonable prospect of achieving a conviction ?
There are other considerations too but this is probably why they are not proceeding in your case.
As Dermot suggests, you can take a civil action but full responsibility for that rests with you as there is no state / prosecution element involved.
You carry the burden of proof which is that of the balance of probabilities. This phrase means that something is at least 51% more likely to be correct than not.
If you lose the case you may expect that an order will made against you for payment of the defendant's legal costs incurred in defending your case.
I think that the present jurisdiction of the District Court is about €15,000. If your total claim is greater it is a Circuit Court case with higher attendant legal costs. If your losses exceed €15,000 you can still take a District Court action but waive the balance of losses over €15,000.
If you are considering proceedings you would need to budget for the costs of an appeal. A District Court verdict is appealed to the Circuit Court by way of a retrial in front of a Circuit Court judge. A Circuit Court verdict is appealed to the High Court in similar fashion except it is heard by a High Court judge alone.
By any chance was the damage to your vehicles covered by motor insurance ? If so, ask your insurers if they would consider funding a case for recovery of those losses from the suspect competitor. The insurers can do this if they have paid for the damage and they consider that there is a chance of recovering funds.
Otherwise, if this is out of your own resources you may have to take the bitter option of suffering the losses. From a business perspective it might be preferable to spend money on improved security instead of a speculative legal action.