Not sure if its been explained yet (as of 21st December 2019), but is:
Tapadh leibh! :-)
Also, when the man is pronouncing this sentence, is sheanair meant to sound like "hea-niv"?
The second syllable is pronounced more like 'nith' in the recording. It's not unusual for an 'r' to become a 'th' in Gaelic, particularly in the Western Isles. In IPA, it would be ɾʲ being changed to a ð.
Ah. That explains it. I always heard Greas Ort locally but previously you said Greasaibh iorbh and it confused me.
why is it not sibhise? it's a grandfather?
Oirbh instead of ort gives the respect/seniority to the grandfather. There's no "thusa" in the sentence, so nothing you'd replace with "sibhse".