There is no deliberate trilling or tapping involved - whatever you're hearing is just just an accidental side-effect of her tongue changing position between syllables.
You can here at least two other examples of comhartha on Duolingo, and many more in the Pronunciation Database at teanglann.ie.
Thanks for your reply. I had already listened to these, and I get what you mean, but accidental or not, in the Ulster example at least, it sounds like a tap to me. That sound happens in English sometimes with the "r" in "thr." I hear it in "uirthi" too. The Ulster speaker in teanglann.ie definitely doesn't do it with "ar" but does with "ar ais."