Christianity which first reached France and Britain was of the school of the apostle John, who ruled the churches in Asia Minor. Colonists from Asia Minor laid the foundations of the pre-Patrick church. They brought with them the doctrine which they received of John, Paul, Philip, and the other apostles of the Lord, which included not only the observance of the seventh day Sabbath, but also the commemoration of Christ's death upon the 14th of Abib--Passover!
"It is probable that the primitive Christians kept the Pascha on the 14th of Nisan as determined by the Jewish authorities, and regarded it as the anniversary of the crucifixion. ...The churches of the Roman province of Asia...followed the older custom, keeping the Pasch on the 14th of Nisan, whatever the day of the week." (James F. Kenney, The Sources for the Early History of Ireland, Vol.1, pp.211, 212; Columbia University Press, New York, 1929)
"...they ignorantly refuse to observe our Easter [Pascha] on which Christ was sacrificed, arguing that it should be observed with the Hebrew Passover on the fourteenth of the moon." (Bede, Historia Ecclesiastica, II, 19 wherein Bede quoted "Pope" John's words concerning the Celtic brethren)
Other doctrines that Patrick, Columba, and the Celtic assemblies held included the observation of the other Festivals of the Eternal (Lev.23), the belief in the mortality of man and the hope of the resurrection (vs. immortality of the soul and going to heaven, hell, and/or purgatory); the distinction between clean and unclean animals; "improvised" prayers (from the heart, rather than merely from the lip with repetitions); that Christ Jesus is our only Mediator--as opposed to various "saints," Mary, angels, etc.; and that redemption and atonement comes through the life, death, and resurrection of Christ alone--separate from works and heeding commandments/doctrines of men (see The Celtic Church in Britain by Leslie Hardinge, as well as Truth Triumphant by B.G. Wilkinson, for documentation).