The Irish influence on the Vita Merlini comes from the Bhuile Suibhne Geilt. The Madness of Suibhne is about a legendary King in Ireland. It seems likely that Henry Blois used this source for narrative ideas. But certainly Henry Blois had access to Irish folk legend because as we have already covered Henry also composes Tristan and Isuelt based on Irish folk lore also.
There are several coincidences mutual to both the VM and The Madness of Suibhne. The madness of Merlin in the VM is slightly confusing. It is not developed as one would expect as part of the narrative. It appears that Henry Blois started with the idea from the Irish source and forgot to develop it. Merlin’s madness comes across as incidental because as we have noted, Henry’s primary aims in the Vita are to provide new prophecies for his audience to puzzle over and to re-educate his readers upon certain facts related in the HRB and to establish flesh on ther new Caledonian Merlin which he had not achieved affixing to the Ambrosian Merlin in HRB. Don’t forget also, Henry Blois is hurting inside seemingly having lost everything and thus he affixes his own internal sadness and ‘Madness’ to Merlin Sylvestris in the VM as well as establishing him in the Northumberland area which funnily enough he contradicts soon atrwriting VM when he composes the John of Cornwall prophecies which by Merlin’s depth of knowledge regarding the topography of Devon he seems to come from that region.
Anyway, a coincidence with the Irish story has Eorann, wife of Suibhne, taking a new mate in much the same fashion as Guendoloena does in the Vita. In the same story we find Suibhne speaking of his herd of stags and then we see Merlin rides one in the VM. Also, Suibhne has his madness softened in a very similar way by Loingreachan who played upon the harp and sang to him of his family, and finally persuaded him to return home just as Merlin returns in the VM.
The story line below from the VM is too close to be Coincidental:
The messenger heard the prophet and broke off his lament with cadences on the cither he had brought with him that with it he might attract and soften the madman. Therefore making plaintive sounds with his fingers and striking the strings in order, he lay hidden behind him and sang in a low voice…. The messenger sang thus to his plaintive lyre, and with his music soothed the ears of the prophet that he might become more gentle and rejoice with the singer. Quickly the prophet arose and addressed the young man with pleasant words, and begged him to touch once more the strings with his fingers and to sing again his former song. The latter therefore set his fingers to the lyre and played over again the song that was asked for, and by his playing compelled the man, little by little, to put aside his madness, captivated by the sweetness of the lute. So Merlin became mindful of himself, and he recalled what he used to be, and he wondered at his madness and he hated it. His former mind returned and his sense came back to him, and, moved by affection, he groaned at the names of his sister and of his wife, since his mind was now restored to him, and he asked to be led to the court of King Rhydderch.