Brendan, I'm not sure about this. I'm not saying it's wrong, of course, but, I think that if I saw the tattoo, I might just translate it as "one day, one time" (i.e I don't think my first thought would be "only one day...").
It seems tricky - and I tend to stay away from anything tricky - but if I really had to have a go, I might go along the lines of "gan ach lá amháin, gan ach uair amháin".

I think this might work but, maybe that's only because I know what I'm trying to say! It might be that others look at it and find it not entirely clear or a bit incomplete-looking?

If so, then ifwe knew the context, maybe we could improve on it by adding a bit more. For example, if the meaning is something like "(We have) one day only" (on this earth, or whatever), adding in an "againn" ("gan ach lá amháin againn") would probably be clearer. Or if it's something like "(we are here for) one day only", making it "gan ach lá amháin anseo muid" might be better, etc, etc.