It has nothing to do with whether or not you fail a speech check. If you fail the speech check he doesn't let you in, but he notices that you're a tribal whether you pass it or not. Here is the successful dialogue track taken straight from the file:
with me. What you want with 'em?}
{191}{}{Me want join.}
{192}{}{I want to join.}
{193}{}{Join up? You're a tribal! What makes you think they'd want a tribal?}
{194}{}{Join up? You're a tribal! What makes you think they'd want a tribal,
and his posse no less?}
{195}{}{Me serious. Me join Enclave.}
{196}{}{Me serious. Me Join Enclave.}
{197}{}{I'm serious. I'm looking to join the Enclave.}
{198}{}{I'm serious. I'm looking to join the Enclave}
{199}{}{Wait a minute,dummy. Are you trying to say you're one of the Enclave
replacements?}
{200}{}{Wait a minute. Are you trying to say you're one of the Enclave
replacements?}
{201}{}{Uh... yeah.}
{202}{}{That's right.}
{203}{}{Damn. Sorry for the hassles. I'm supposed to clear a bunch of newbies
coming through here but I got to weed out the trash. Anyway, just go up to the gate,
through the woods there, and give 'em the password. It's 'sheepshead.' Oh, and stay on the path!}
{204}{}{Thanks!}