The Limerick literary festival will include a limerick competition. There's a €500 prize and it's free to enter. The 'rules and guidlines' contain useful information about the structure of a limerick and tips on crafting a good one.
The only very slight snag is that to compete in the final, you'll have to go to Limerick.
I'm not suggesting that going to Limerick is in any way a bad thing, but the date might not be convenient for everyone. I'll be elsewhere at the time myself, so can't enter. That's a pity, because I've already made a start. Perhaps you'd like to help me finish it?
There was an odd writer with a campervan
Whose poems hardly ever properly scan