Christopher Shultz wrote a useful review on screenwriting software for IOS and Android tablets and smartphones.
The five apps he covers in the post are scriptly, Fade in Mobile, Celtx, Scripts Pro and Final Draft Writer.
Although he just mentions it in passing, DubScript for Android looks also great given its high ratings in Google Play.
Read the review here.
Or you could use a free screenplay template for Apple Pages (works between Mac and iOS devices) compiled according to Final Draft’s Cole & Haag standard. Go to http://story-architect.com/?p=3397