The only instance in regard to which I have heard "theorising" of Spanish influence on Irish "folksong" is in relation to An Fhallaingín Mhuimhneachaín (the Munster cloak) which has been thought to display Spanish influence in its form.

You will find many references to Spain, but most of them will be trite (as you imply, for rhyme rather than reason) and of a secondary nature. Other references, although romantic, can be evidence of the Irish consciousness of Spain at certain times. The reference in Róisín Dubh would be the most prominent example there, the "allegorical interpretation" implying that Spain and the Pope will come to the aid of Róisín (Ireland according to that interpretation). Even then, the interpretation is most probably evidence of later backstorying rather than contemporary attitudes at the time of the song's composition (also in itself later than the historical period to which it is, in that interpretation, presumed to refer).

You should beware the superficial level of many references. As I grew up I was aware of Iníon Rí na Spáinne (The Daughter of the King of Spain) as a classic "damsel in distress" who was saved from danger by the Irish hero (and lived happily with him ever after) in tales told by storytellers locally. She could just as easily have been the daughter of the king of the western world or of the king of Syria who had the same role in other stories.

You should also beware the danger of interpreting references to Spain in English-language songs in Ireland as direct - many could be clichés carried over in imitation from English compositions rather than evidence of indigenous attitudes.   The same is true, but to a lesser extent, of material in Irish.

And unless you study the Irish-language material, I am afraid you will be attempting to find the influence of red and yellow on the paintwork of a green car while having only the tyres for evidence.

There would seem to be many pitfalls to the research you propose and it may be wise to consider that the lack of an existing book may be due to those issues. A compendium or compilation may be possible but what is its extrinsic worth? It could also suffer greatly in comparison to work in other areas of Hiberno-Spanish contact.

There has been a flourishing in the amount of work being carried out by academic historians and literary researchers, in both Spain and Ireland, on the intellectual contact during the late sixteenth and early seventeenth centuries which was extensive. Most of the extant literary production from that contact is in prose but you could include a fairly large body of verse if you were to expand the boundaries to the production of Irish poets in the Spanish Netherlands during the seventeenth century. One poem from that milieu has been discussed here in other threads - Dia do Bheatha a Naí Ghil.   To say that there is a Spanish influence because of that tenuous connection would be stretchin things (however the author had previously spent time in Spain and had received some of his education at the University of Salamanca).

There is also some fairly recent work by economic historians on the contacts during a later period between the Irish and the fisheries along the northern coast of Spain. I don't recall any reference to song or poetry in those discussions (nor indeed any references to the contacts in Irish song or poetry).

You can go all the way back to the seventh century for real historically evidenced contact. The work of Isidore of Seville was apparently known in Ireland before other parts of Europe (as far as extant evidence shows) and his magnum opus, the Etymologiae, was known in Old Irish, fondly, as the Culmen. The Irish annals also carry reports of the arrival of Vikings on their return from Seville - 9th or 10th century according to memory - and there are some grounds for speculating that they were Irish-based Vikings.

All in all, I would think that if you have a few years to spare for research, and wish to spend them on Irish-Spanish contact, there are more viable options.

But as for the compendium (which might show that there is in fact profound material), my initial contribution is:

An Fhallaingín Mhuimhneach(aín)
Róisín Dubh.


PS

(Bill Whelan also composed a suite at the time of EXPO in Seville - 1992? It used to be available on the Tara label and some of the ideas in it were later developed and included in Riverdance - the Irish show with Flamenco and Harlem influences.)