I think you should find a book of modern mythic retellings for both cultures. For Myths: Celtic myths come from several different celtic cultures, so in a good collection you would get some welsh, some irish, and maybe scottish and manx. THEN after you have read some modern retellings, try cracking into primary sources like the Eddas, Mabinogion, etc.... You can also read some of the more modern neopagan works, but I am out of the loop enough to be at a loss for recommendations. There seems to be a few Asatru ones, but few Celtic ones that are not heavily influenced by Wicca and Mainstream neopaganism ( which is cool I guess if you are into that).
The suggestions I am giving you are from the ADF website. I was a member of ADF for a long time before I started crafting a personalized neopaganism that broke all of the reconstructionist rules.
Here is a list of primary sources ( from several in the Indo-European language family, but Celtic and Norse are included ). http://www.adf.org/training/resources/primary-sources.html
Here is a list of readable mythology books: http://www.adf.org/training/resources/mythology.html
You can also read some books on mythological "theory" that are specific to the cultures you are interested in: Celtic Culture by Alwyn and Brinley Rees, and works by Hilda Ellis Davidson for Norse. Anyway, Like I said, I think the best thing for you to do is to read a collection of readable mythology books for the two cultures, and spend some time every day working through some of the primary sources and historical works.