i am will be in thailand for 38 days, being from ireland i qualify for the 30 day visa on arrival, i have been told that if you can prove you are leaving thailand within 45 days of arrival you will be given an extra 15 day stamp, is this true?

It is a visa exempt entry and you will only get 30 days. A visa on arrival is something completly different.

If you don't have a ticket out of the country within 30 days the airline may not allow you on the plane.

If you need more than 30 days you should get a tourist visa from the Thai consulate in Dublin.

Website for consulate: http://www.thaiconsulateireland.com/

If you are not in Ireland but in the UK contact the consulate in Hull. You will get your passport back within a few days if applying by post. http://www.thaiconsul.co.uk/

As said you may not be allowed on the flight without a Visa.

If you did make it to Thailand the most you would get from Immigration would be a 7 day extension.

The answer to your question is no. There is no extra 15 days. If you had a ticket out within 7 days you could pay 1,900 baht for an extension of stay that lasts 7 days.

Perhaps you are thinking of the new 15 day visa exempt entry by land. You could leave/return to obtain a new 15 day stay with a border crossing - but day of entry is day one of the count.