"he is unreal" would usually be ta sé dochreidte.
"it's not real" or "that's not a real X" can be expressed with fíor (níl sé fíor or níl X fíor é sin or níl fíor-X é sin). "He is not real" sounds a bit odd to me, though you might say it to a child if you wanted to rob them of the magic of Santy!
Thanks! I should have been a bit more precise, though:
Ní is actually the general negative marker for all ordinary verbs, as well as functioning as the negative of is (we're basically referring only to the present tense here, by the way).
So you'd say tuigim "I understand" but ní thuigim "I don't understand", as ní lenites.
The reason it seems to be different for tá is because of the way that tá changes to "fuil" in certain places, as in an + tá? = an bhfuil?. Well, the same thing is happening here, except ní lenites, so we end up with ní + tá = ní + fuil = ní fhuil. The combination fh has no sound at all, so ní fhuil is always written and pronounced as níl. So níl is really just a contracted form of ní with tá.