Healthcare for Irish citizens

The same as everyone else and it would depend on their age. If over retirement age and receiving a state pension in their own country or registered disabled or certain other things, they can get an S1 form which transfers their health care to Spain and is paid for by the country that pays their pensions 

If not eligible for an S1, they would require private health insurance with no co-payments that cover them to at least the Spanish NHS standard. After one year of residency, they can apply for the convenio especial which puts them on the Spanish NHS regardless of existing problems. That would be 60 euro per month if under 65 and rising to 157 euro a month if over 65. That cost does not cover prescription costs.

Here is a link to all the requirements for Irish passport holders to live in Spain. Your friends will initially need to have private healthcare then after a full year of living here legally they can apply for the Convenio Especial healthcare which has monthly fees. If they are of retirement age and receiving government pension they can apply for healthcare through the S1 healthcare scheme which is paid for by their government. 

https://upsticks.es/residency-in-spain-for-irish-passport-holders-all-you-need-to-know/

Steve 

Have they reached Irish State Retirement age? If so, they can transfer their healthcare to Spain by obtaining form S1 from the Health Service Executive. If not , they would need private health insurance for the first year. The health insurance must cover all eventualities including pre-existing conditions and without copayments. Once legally resident in Spain for 12 months they can apply to join the Convenio Especial. The Convenio Especial is contributory for a monthly fee (€60 under 65, €157 over 65) and will provide full access to the Spanish healthcare system including pre-existing conditions. However, prescriptions must be paid for in full.

These links will provide more detailed information:

https://www.ireland.ie/en/spain/madrid/services/new-to-spain/

https://upsticks.es/healthcare-in-spain-what-is-the-convenio-especial/

https://www.gva.es/es/inicio/procedimientos?id_proc=17044

https://www.ageinspain.org/moving-to-spain

https://age-in-spain.helpscoutdocs.com/article/18-i-want-to-live-in-spain-but-i-m-not-yet-a-resident-can-i-access-spanish-healthcare-once-i-become-a-resident