Rainey is mainly found in eastern Ulster - Antrim and Down.
There can be several origins of the name:
1. It may denote descent from an Irish family, O Raighne.
2. It can of English or Scottish origin. In County Down Rainey was interchangeable with the Scottish name Rennie.
3The name Reaney is an anglicization of O Raighne.

As far as a family creat is concerned it is important to check your name's origins because there could be a variation in them. You should check in a library for books on Irish, Scottish and English heralry and see what the designs look like. There are many commercial companies who will provide you with a copy of a crest but it is important to make sure they are authentic and not, as is common, a made up one.
You could try <familycrests.com.r> or <iol.ie/~plugin/herald01> If you type in rainey family crest on one of the search engines such as Google you will find many of these. Flaunches or flanches are bites taken out of the sides of heraldic shields. Mnay so called "family crests" were simply made up by Victorian families to impress others and have little or no historical significance. So check authenticity before you buy.