Shy but willing, like an ass to a thistle
(if you've ever watched a donkey approaching a thistle, ears laid back, eyes scrunched closed, lips carefully drawn back from teeth, but with somehow an expression of expectant ecstasy...)

As crooked as a ram's horn

Crooked? If he et a nail he'd shite a screw!

Telling lies as fast as a horse could trot

As thick as a kish of brogues
(=basket of shoes)

Not as green as she's cabbage-looking

Tuppence-ha'penny looking down on tuppence
(this surely needs amending into euro)

Some of these are metaphors, but hell, it's all in the family, isn't it?