How do you pronouce Gaelic word tiermann?

There's no single "Gaelic" language. "Tiermann" could be from Irish or Scottish Gaelic, and the pronunciation would differ slightly. Without knowing which Gaelic language is intended, it's impossible to give a definitive pronunciation.

However, assuming it's Irish Gaelic, the pronunciation would be roughly:

TYER-man

* TYER: The "ie" makes a long "i" sound, similar to the "i" in "tie".

* man: Pronounced as in English.

If it's Scottish Gaelic, the pronunciation could be slightly different, potentially with a shorter "i" sound or a different stress pattern. More context is needed for a precise pronunciation in Scottish Gaelic.