I incurred a 10-11 year old IRS tax debt that I am paying off monthly.
I am curious whether getting married or getting my girlfriend on my insurance as a domestic partner will affect her credit et al with the IRS and my tax debt
Also I have heard that there is a time limit for the collection of a tax debt with the IRS of 10 years. Is this correct if I am on a monthly installment plan?

If the IRS has agreed with the installment plan, you will be fine.

As to your girlfriend, more than likely she will be able to file as an, "Injured spouse" should she choose to marry you.

Of the may things I've heard the IRS called, I've never heard the term, "Credit" used.

Contrary to popular beliefs, the IRS is actually pretty decent about helping taxpayers. But they do report on offenders.

Don has covered all the pertinent issues.

Your girl friend/soon-to-be wife can either file the Injured Spouse form to not be linked to your tax situation or, better still, just file Married Filing Separately.