Yes you get 25% on the standard fare (you have to book at a ticket office for example in Interlaken)

Before booking i recommend to check the weather it´s still quite expensive to get up there and worstcase you see only clouds  and wasted the money :/

Thanks a lot for your quick reply. And yes your suggestions are valid and noted. 

Yes, you get 25% off with a Eurail pass and, very important, you do not need a travel day.

May I know, when I use this discount that means my Eurail pass must be active on that day? Asking just to know the benefits of Eurail pass before purchase

No, if you're just getting a discount, you do not need a travel day.

As @rvdborgt  already said no active travelday needed (few days ago a traveler had a issue there as the conductor thought he need a travelday )

BUT to reach the starting station you most likely have to use other swiss trains that are on IR and hence need to validate.

NOTE: untill 11/12 there is a 25% discount on the SWISS travel pass-this is about the same as IR, but only valid in Switzerland, also on local bus/tram-incl. the yellow post buses and also gives-sometimes more-discount on the many mountain climbing railways. The fare to Jungfrau is among the highest in the world for the distance-even with the discount.

Thanks for your note.. yes I have to use my pass on day1 and I want to visit Jungfrauhoch on day2 from interlaken… so If I am correct, i don’t need to use IR pass on day2 from interlaken to jungfrau after purchasing a tocket to jungfrauhoch, correct?

That is correct.