What you need is called "Angle Iron" and you will get exactly what you need in Butler's Steel in Ballaghaderreen. It is expensive, as it is galvanised.
Your stone mason should know someplace that sells it, as he would be using these all the time. He should also be able to tell you the size and the gauge needed, as this will depend on the width of the opening, and the amount of stone that the lintel will have to carry.
Get it organised as soon as you can, because it can often happen that there is plenty of steel in stock, but is not galvanised, and you will have to wait until it has been done.