"You're arrested for Gate-breaking, and Tearing up of Rules, and Trespassing, and Bribing Guards with Food."

Shire Shirriff

Vigilance
When Shire Shirriff enters, you may sacrifice a token. When you do, exile target creature an opponent controls until Shire Shirriff leaves the battlefield.

The Lord of the Rings: Tales of Middle-earth

Vigilance
When Shire Shirriff enters the battlefield, you may sacrifice a token. When you do, exile target creature an opponent controls until Shire Shirriff leaves the battlefield.

If Shire Shirriff leaves the battlefield before its last ability resolves, you can still sacrifice a token. However, when you do, the target creature won't be exiled when the "reflexive" triggered ability resolves. Similarly, if Shire Shirriff leaves the battlefield while the "reflexive" ability is still on the stack, the target creature won't be exiled.

You don't choose a target for Shire Shirriff's last ability at the time it triggers. Rather, a second "reflexive" ability triggers when you sacrifice a token this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.