There’s no link between them.
The German verb fragen is thought to go back to Proto-Germanic fregnaną and Proto-Indo-European preḱ- (both of these are theoretical reconstructions). Irish iarr is cognate with these — the dropped initial P in some Irish words can also be seen in other cognates, e.g. íasc vs. Latin piscis.
The Irish verb freagair is thought to be derived from a theoretical Proto-Irish fris-gairid, roughly “against-calls”. Fris is cognate with Latin versus; it probably corresponds to freas- in the modern language. I don’t know how Proto-Irish gairid compares with other languages; its modern Irish descendant is gair.
There is an old but classic Irish etymological dictionary - Alexander MacBain's. It might not be what you want as a learner, because it is primarily in Scottish Gaelic, but it also lists Irish and other Celtic headwords. Here's a link to a free online searchable transcript.