I've read this in Cnósach Focal ó Bhaile Bhúirne:

This is exactly as given in that source.
Cuir leis here should mean that there are enough of them to support or back up in some way. What does this mean? We only have the collective voice that we collectively can support or back up? This, with no further context, doesn't have a clear meaning, to me at least.

Cuir le can have the meaning "to support", as I'm sure you know already. See number 6 here, Cur le chéile "to give mutual support". Does your sentence not simply mean "we have no voice but voice that we are willing to support", or something along those lines? "We have no voice but the voice that is supported by us", perhaps?

Thank you. It must be something like that. Seasamh 'na bhun - does this also mean "support"?

I can see how it would purely idiomatically, "to stand at its base", like a pillar I guess. But, it's not an expression that stands out in my memory as something I've encountered before, so I can't say whether it's a set phrase or not.

Thank you, Ade.