Where does “Ольстер” come from?

Ольстер (Russian) comes from English Ulster, from Old Norse Uladztir, from Old Irish tír, from Proto-Celtic tīros, from Proto-Indo-European ters- — dry.

Ancestry of “Ольстер”, step by step

Step	Language	Word	Meaning
1	English	Ulster	Alternative form of ulster
2	Old Norse	Uladztir	Ulster
3	Old Irish	tír	land, country, territory, ground
4	Proto-Celtic	tīros	dry land
5	Proto-Indo-European	ters-	dry

Every word from Proto-Indo-European ters- →Every word from Proto-Celtic tīros →Every word from Old Irish tír →