just a technical note: patrick is supposed to have lived, and evangelized
ireland, in the early 5th century (c. 435); the policy of building churches
on ancient pagan sites and assimilated pagan deities as christian demons was
promulgated by pope gregory the great in the late sixth century (c. 540-604).
patrick was working within the framework of the gaulish and primitive welsh
church; gregory and augustine were working within the roman church; the two
systems were not officially reconciled until the Synod of Whitby in 663, and
it took some time until the whole irish church accepted the reforms of that
synod. therefore, while the irish church may have come up with the same idea
of utilizing pagan mythology for christian ends, i think it's a mistake to
assume that what one bunch of christians were doing in evangelizing the
anglo-saxons is applicable to what another bunch of christians were doing in
evangelizing the celts.
certainly there was mutual influence between celtic paganism and celtic
christianity. just he fact that the people who were becoming christians
lived in an environment shaped by the assumptions of the previous world-view
must have shaped their understanding of what christianity was all about.
furthermore, i think that the enormous influence of irish monks in creating
what later became pan-christian theology and dogma has been underplayed by
many scholars. but i think it's important to keep the chronology of the
spread of christianity clear, and not to assume that the religion was as
monolithic in the early times as it became in the later middle ages.