Dogs Trust will subsidise the neutering of dogs for people who are in reciept of a Means Tested Social Welfare payment... The only cost to the owner is €20...
You just ring them to register... They'll give you contact details of a vet in your area who's taking part in the scheme... You then ring the vet to make the appointment... Then on the day, you take the pooch to the vet's, along with proof of eligability (SW Payment card, or payment book), and €20, and the vet does the rest...
The telephone number for Dogs Trust is: 1890 946336.
For other pets, I suggest getting in touch with your local SPCA to enquire...