The Irish name for a shamrock is "seamróg."
The name Rachel is spelled the same in German.
It is the shortened form of Uilliam (Irish for William).
No. It's a modern combination of the popular elements of Shay- + -Lyn. Shea/Shay is an Anglized version of the Irish name Séaghdha, but just adding the English -lynn to an Irish name doesn't mean it's still Irish.
I don't know of any Irish language 'equivalent' of Jillian.
Rachael is not an Irish word so it has no meaning in that language.
In Irish Gaelic: Ráchael Nic Cormaic or Ráchael Mhic Cormaic (married name),
Rachael Robbins's birth name is Rachael Robbins.
Rachael Thompson's birth name is Rachael Martin.
Rachael Ray's birth name is Rachael Domenica Ray.
Rachael Stirling's birth name is Rachael Atlanta Stirling.
Rachael McAfee's birth name is Rachael Copeland McAfee.
Rachael MacFarlane's birth name is Rachael Ann MacFarlane.
Rachael Kahne's birth name is Rachael Cecelia Kahne.
Rachael Everett's birth name is Rachael Maree Everett.
Rachael Houston's birth name is Rachael Leigh Ethridge.
Rachael Flatt's birth name is Rachael Elizabeth Flatt.