How can I calculate my ovulation day if my period is irregular. E.G my cycle was
19 days by the way Aug and Sept
27 days by the way sept and oct
30 days by the way oct and nov

I'm not sure this will work... just keep track of the first day of your last period (since it is irregular) and count 14 days after your first day. I WOULD THINK it would, no matter what, be 14 days after the first day of your last period. So just keep track!

A guide for when periods are irregular is to subtract 18 days from your shortest cycle and 11 days from the longest. This is the time frame you are most likely to ovulate in. It will be a fairly wide range given the differences in lengths of cycles. Generally you can assume you will ovulate about 12-16 days before your next period is due, regardless of how long your cycle is, but that is for someone with a regular cycle.
Of course, plenty of women don't follow any of the norms and will ovulate sooner or later than the norm on a regular basis.
Think of them as a starting place and also watch for possible signs your body is preparing for ovulation so that you can adjust accordingly.