The only tiny, yellow fungus that I know of that looks like this is the delightfully named Lemon Drops (Bisporella citrina), but there could be others, so my ID is only tentative. You need a hand lens, or of course a macro lens (as in this photo), to see this attractive sac fungus. It has a maximum cap size of about three millimeters. I found this display on a rotting log in North Glenmore Park on 6th September.