Yes. Ní (with lenition) negates verbs: Ní ritheann sé: "He doesn't run".
Now tá means "is, are", but when it follows certain particles such as "an?" and "ní", it changes to the form "fuil", which never appears exactly as "fuil" because of the effects of lenition and eclipsis. With "an?" you get an bhfuil? = "is it?", but when you negate tá, you originally got ní fhuil. Since fh has no sound, this is always pronounced and written níl.
So níl = "it is not".
So Níl capall agam = Ní fhuil capall agam (although this expanded form is not used) = Ní + Tá capall agam.
(I corrected this comment to clear up a silly mistake).