I seem to have read somewhere that the grammatical structure of english is celtic and so though english is a germanic language essentially it is one spoken by a non native speaker. Does anyone know if this is true?
No. English was brought over wholesale by the Germanic Angles and Saxons(the English). It's actually amazing how little Celtic influenced English.
There are two cases of grammar usually brought up as being due to Celtic influence, 'do-support' and the 'progressive' tense. But the timeline works against the theory, which leads to eccentric reasoning to make it work, and linguists don't generally accept the claim at all.
I have never heard this claim before, and from what I know of modern Celtic languages, the grammar is not very similar.
This would probably be a better question for /r/asklinguists