Where does “seasaigh” come from?

seasaigh (Irish) comes from Irish seasamh, from Old Irish sessam, from Old Irish sissidir, from Proto-Celtic sista-, from Proto-Indo-European stísteh₂ti, from Proto-Indo-European steh₂- — to stand.

Ancestry of “seasaigh”, step by step

Step	Language	Word	Meaning
1	Irish	seasamh	verbal noun of seas; standing, a standing...
2	Old Irish	sessam	verbal noun of sissidir: standing; standing by,...
3	Old Irish	sissidir	to stand
4	Proto-Celtic	sista-	—
5	Proto-Indo-European	stísteh₂ti	to be standing up, to be getting up
6	Proto-Indo-European	steh₂-	to stand

Every word from Proto-Indo-European steh₂- →Every word from Proto-Indo-European stísteh₂ti →Every word from Proto-Celtic sista- →