Figured I'd share some interesting citations on the topic, feel free to share whatever you may know on the subject as well.

From The Greatness and Decline of the Celts by Henri Hubert discussing general ancient Celtic influence on the Germanic people:



A similar overview from the book The Baiuvarii and Thuringi: An Ethnographic Perspective

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A citation regarding the Celtic origin of Germanic warband practices and the cult of Odin, as well as the same influence of the Celtic comitatus on early medieval feudal structure from Michael Enright's Lady with a Mead Cup: Ritual, prophecy and lordship in the European warband from La Tene to the Viking Age:



A discussion on the Celtic intermediary of the runic script (i.e. the runic script may not have been Celtic in origin ultimately but they may have been the vehicle by which it was appropriated by the ancient Germanics) from the article The Celts and the Origin of the Runic Script byBernard Mees:



Another area of interest albeit later in time is the possible Irish influence on Icelandic literature and thus some of the motifs of Norse mythology. To take two examples, the Rígsţula is known to be indebted to Celtic conceptions and likely the myth of Odin at Mimir's Well also. Here's an article that goes a bit more in depth as a starting point for those interested:

http://www.aughty.org/pdf/sagas_iceland_ireland.pdf