I read the information on the Spanish government website differently. It could be presented in a clearer way than it is the case.
If you travel by land from France to Spain:
- You do not need to fill in the SpTH form and you do not need to get the QR code associated with it.
- There are no public health related controls at the land border between Spain and France.
- Currently (27 July 2021) both all of Ireland and the regions of France that share a border with Spain - Nouvelle-Aquitaine and Occitanie - are declared as risk areas by Spain. See list of risk countries/risk areas here. The list gets updated every 7 days and will change until September when you plan to travel.
It is my understanding that travellers from risk areas/risk countries must have Covid-19 related documentation, see Spanish government document about
NEW HEALTH REQUIREMENTS FOR ENTRY INTO SPAIN AS FROM 7 JUNE. The relevant quote is this:
F. What if I enter Spain by land?
All people aged 6 years or older from countries or areas at risk arriving in Spain by land must have one of the documents (certificate of vaccination, diagnostic test or recovery) specified in the first section of this document.
If you are fully vaccinated and as you are from Ireland, the EU Digital Covid Certificate would be all you need to enter Spain from France and you will not be controlled.
BTW, as to passports: The rule for nationals of non-EU countries who want to enter the Schengen area says that their passport must be
valid for at least 3 months, counted from the day they plan to leave the Schengen area. As you are the national of an EU country, this rule does not apply to you. I would be surprised if a stricter rule than what I just quoted applies.
Buen Camino! And apologies for the length of the post.