What would make a circle cloud like this?

Forgot to mention,you really CAN see natural "smoke rings" in the Earth's atmosphere.

Mount Etna regularly entertains visitors by blowing smoke rings.

(Don't know what Mount Etna will do when they ban smoking in public places in Sicily.)

See:
news.bbc.co.uk/1/hi/sci/tech/696953.stm

According to the news there was a mini tornado in east tipperary 2 days ago, so maybe the weather is acting even more strange then normal.

Thats called a lenticular cloud, was it near a mountain? Its a sub class of orthographic clouds which usually form when air is forced over a mountain barrier creating a series of waves that stretch downwind. They often appear to hang for lenghty periods in the same spot. That one looks to me like a particular subbreed of lenticular cloud known as 'altocumulus lenticularis' they are usually more almond shaped than round but can be round too ass seen in your great photo.

Lenticular clouds can still form tens of miles away from mountains in the direction the wind was coming from.

Thats called a lenticular cloud, was it near a mountain?

No mountains near here, just a couple of low hills surrounding the upper broadmeadows estuary. I suppose the Howth, Ireland's Eye and Lambay hills might be high enough to stir things up a little. I have seen some photos of lenticular clouds near mountains in New Zealand. Amazing stuff!