(BTW, I'd think anyone with such a clear tie to the ancient Christianity of Ireland would have a better understanding of the ability for people to disagree on calendrical issues with total good faith on *both* sides).
Not sure about the good faith on both sides, alas.
The incoming Roman missionaries refused to recognise the consecration of the existing Irish and Welsh bishops and reconsecrated them. They also did large scale rebaptisms. From their side the Celts refused to recognise the Roman Sacraments and refused to eat off plates which had been used by the Roman missionaries. There was not much "ability to disagree" on either side.
Neuman, Carol, The NorthUmbrian Renaissance, Associated University
Presses, N.J., 1987, ISBN: 0-941664-11-2, p. 58 -
"Elsewhere, however, matters were not so benignly worked out. Theodore
of Tarsus, on his arrival (as archbishop of Britain) in 669, found it necessary
to use forceful measures to quell the remnants of the Celtic heresy. Despite
the direct and immediate effects of Whitby on the central Celtic house at
Lindisfarne, it may be remembered that the Picts and Scots, including at
this point the Columban motherhouse at Iona, remained unwilling to
accept Roman orthodoxy. Theodore's 'Penitential' clearly announced his
views on the issues. He recognised neither episcopal consecration
nor baptism as performed by the Celtic Church. Eddius tells us that he
insisted on reconsecrating Chad, "through every episcopal grade," and
demanded the rebaptism of converts of the Celtic Church. He also
ordered a year's penance for anyone receiving communion from Celtic
clerics. The hostility along the Welsh and Cornish borders was
apparently mutual. Aldhelm of Malmsbury wrote that the Welsh bishops
considered the clergy of Rome to be excommunicated until they should
individually perform forty days penance, and refused to pray with them
or join them at meals. The leftovers of food touched by Roman clerics
were ordered thrown to swine so that Celtic Christians would not suffer
spiritual contagion. Their vessels were to be purified with fire or san,
and they were to receive neither salutation nor the kiss of peace.
Apparently the British had not forgot the lessons of St. Augustine's