Remember that the possessive pronouns also make the noun definite. Mo chara means, not “a friend of mine” (that would be cara liom) but “my friend”.
Once there was an example like this in Comhar: shíl siad gur guth ár ndúchais é Oscar Wilde. This is wrong. Correct possibilities are: shíl siad gurbh é Oscar Wilde guth ár ndúchais (“they thought that O.W. was THE voice of our roots/identity/native culture”) and shíl siad gur guth dár ndúchas é Oscar Wilde (“they thought that O.W. was A voice [speaking] for our roots/identity/native culture”).
Syntax is important. Pay attention to it.