Hello Joe. You are correct in stating that the Irish Catholics were largely like their European counterparts in most things, including the language of the mass. To understand the historic uses of the Gaelic mass, so one would study the history of the Irish language revival and romantic nationalism, not church history.
That said, there are some differences in the Irish Catholic folk religion, which I alluded to above. For instance, from the Christianity Today website on Celtic Christianity, it says:
"Far from being culturally and religiously isolated from Europe, the Irish and Welsh prayed in Latin for most liturgical purposes, just as their Christian brothers did throughout the Western Church. We still have several Celtic manuscripts in which the prayers of the Mass, baptism, and anointing of the sick are recorded—all quite similar to those in other parts of the European church...Alongside the psalms, biblical canticles (such as the Magnificat), and hymns—both Latin and vernacular—were popular.
Vernacular prayers—we have more in Gaelic than in Welsh—were less ecclesiastical in feel. They reflect a more personal or domestic use. Such prayers include praises of God, prayers to his saints, requests for protection, and blessings. Some even seem more like magical charms than prayers. Of course, there was also a great deal of what we might now call "folk Christianity"—the faith of a largely peasant population—as well as native poetry and lore.
There were differences in detail between the Celtic Christians and their continental neighbors: church architecture, Easter dates, inheritance laws, and local traditions. But almost all the main features of early Celtic Christianity could be found anywhere in Catholic Europe, where every tribe and tongue and nation made the gospel their own."
While the mass would have been said in Latin, as elsewhere, the people were ministered to in their native language. The prayers, etc. discussed above show that since medieval times, there has been a strong Catholic Gaelic folk tradition. Many Gaelic hymns were composed by women, which are traditional. However, most Irish traditional musicians would not accept these as "traditional" in the sense that secular music is "traditional" even though they both are clearly traditional.
As to when the first Gaelic mass was said, I don't have the answer to that one for you. The answer probably lies somewhere in the pages of Gaelic League history, I'd guess.
That said, if you do a google search using the keywords "Gaelic mass" you might be surprised at what turns up.