What about ovulation with an irregular cycle?

The best thing is to use tests that also show a faint line; this indicates that you're close to ovulation.

Unfortunately, there's no specific test for irregular cycles. The ovulation test shows positive when your levels are high, meaning you're most fertile. This works the same even if you have an irregular cycle.
What I would do (and will do soon) is buy cheap strips and test every other day. This increases the chance of catching your peak level and still gives you enough time to try for pregnancy.