|Born||9 April 1981|
|Education||English Literature and French|
|Alma mater||Trinity College, Dublin (TCD)|
In 2001, she was runner up in Raidió Teilifís Éireann's The Selection Box. She joined TV3 as a weather presenter on Ireland AM in 2004. She has presented on Xposé, a program on TV3, since Xposé launched in April 2007.
In 2013 she married her businessman boyfriend of six years John McGuire in Sicily. In June 2014 she announced that she was pregnant and a baby boy Finn was born in December 2014. In October 2015 she revealed that she was five months pregnant with a second baby due in March 2016.
- "Baby on board: Overjoyed TV3's Karen Koster announces she is expecting a baby with husband John on New Year's Eve '". evoke.ie. 30 June 2014.
- "It's a boy! Xpose's Karen Koster and husband John welcome baby Finn'". evoke.ie. 1 January 2015.
- "Baby's FIRST picture! Proud parents Karen Koster and John McGuire share photo of son Finn". evoke.ie. 2 January 2015.
- "Next year little man!' Karen Koster jokes Finn is 'peeved' he didn't get a VIP Style Award". evoke.ie. 28 April 2015.
- "My boobs shrunk since having a baby' says TV3's Karen Koster". evoke.ie. 4 June 2015.
- "The new boss? Karen Koster brings baby Finn to work". evoke.ie. 23 June 2015.
- "Irish twins? TV3 star Karen Koster reveals some BIG pregnancy news". evoke.ie. 19 October 2015.