if you want to have a same alert dialog ui on android and ios,you can use react-native-alertui,it provide a common component for both android and ios platform.
cd example && npm install
npm install react-native-alertui --save
import AlertUI from 'react-native-alertui'
now you can use AlertUI maybe with the code below.
<AlertUI
title = 'Alert'
message = 'this is a message from alertui'
buttons ={
[
{
text:'OK',
onPress:function(){/*dosth*/}
},
{
text:'Cancel',
onPress:function(){/*dosth*/}
}
]}
/>