Didn't the Celtic Church have beliefs that were ultimately heretical in the eyes of the Roman Church?
The Irish Church of late antiquity and the early middle ages was considered to have maintained some beliefs which were at variance with the teachings and practices which were then current in the Church of Rome, most famously regarding the calculation of the date of Easter and certain ritual and liturgical details relating to the celebration of Easter and details of monastic rule. The issue was intended to be solved by the Synod of Whitby at which Bishops and Abbots from Ireland and from Irish foundations in England met with Saxon Bishops and Abbots whose tradition came from the Roman catholic mission of Augustine.
The north of England had been evangelised by the Irish and much of the south had been converted by the mission of Augustine, so the Synod of Whitby ultimately turned into a clash of authority between the Irish bishops and the Saxon bishops who wished to extend their control northwards.
The Irish bishops and abbots largely rejected the Roman catholic revisions of the Saxon church probably on the basis that acceptance of them would have placed their English foundations under the overall control of the Archbishops of Canterbury. Whether the liturgical differences between the Roman Catholic Saxon Church and the Irish Church were even the most central issue at the Synod of Whitby is disputed, and it's thought that these differences were magnified by Saxon bishops who sought to discredit Irish Christianity as part of their attempt to rest control of the northern English congregations from Irish bishops and abbots.
The Roman catholic church, by which I mean the Papacy itself, never outright condemned the Irish church as heretical, though it did roundly and vociferously condemn its peculiarities.
The impression that the ancient Irish church was heretical suited the early Saxon bishops who wished to align themselves with the Roman church more adroitly and later it served the Anglicans who formed the "Anglican" Church of Ireland who could then more thoroughly appropriate these supposed ancient heretics as the forefathers of their faith.
But the Irish Christians of late antiquity and the early middle ages were never officially condemned as heretics, no.