The way the Táin uses poetry is interesting to me. We see it at very important/intense moments - Dubthach's chant, the Morrígan's words as the army cross the plain, Fer Diad and Cú Chulainn's fight, etc. Which you would expect. But, as Carson points out, the roscada are often used during very tense exchanges, as "verbal jousting or an exchange of veiled threats."
And I know nothing whatsoever about Old or Middle Irish and have not read much scholarship on the Táin, so I am very much speculating here, but I think at certain points in the text something like this might also be going on with the poetry. Because the other context in which we see poetry used in the Táin is in dialogue when characters are deceiving each other.
It's used when Medb suspects Fergus of misdirecting the army (which he is.) And it's used when Medb persuades Fer Diad to fight Cú Chulainn, after she gets him drunk and tells him Cú Chulainn insulted him publicly. Cú Chulainn and Fer Diad also exchange verse dialogue before fighting. Which makes sense, given that it's a significant/dramatic moment - but it's also a moment when they are taunting each other, and trying to conceal their unwillingness to kill each other with bravado.
I think when Táin characters exchange dialogue in verse rather than prose, it's sometimes to signal lying/concealment/artifice. In the instances I pointed out it happens when characters are very carefully considering their words, and choosing what they will say for the calculated effect on the listener. And to me at least the poetry adds to that sense of tension, of speech that is being carefully managed.
