The following applies if you require using Yoma/Realex payment method in Iframe. Payment method code visible in frontend is 

pmethodrealexredirect

As per page 14 of Realex configuration guide:

https://developer.realexpayments.com/guides/pdf/Realex_Integration_Guide_Magento_V1.1.pdf

Merchants can use the iFrame functionality, however due to incompatibility with the OneStepCheckout extension, the iFrame cannot be displayed on the checkout itself. It must be displayed on an interim page.