You can make fiber IRUs stick even if the company who bought the fiber
goes belly up.IRUs ("Indefeasible Rights of Use" seems to be the acronym?) as far as I
know, is just that, you actually own the fibers you IRUed for the time
being.
As with everything in life, it will always depend on the contract you have
with your supplier and the contracts that you supplier has with this
suppliers and so on.
For example if Bozotelco builds a huge fibre network all over the
world but then goes bust before they paid the bill for the fibre
- who owns the fibre ? 
Regards,
Neil.