To get less opaque red
1) choose dark roast (roasting process is what gives you the red color and also the roasted flavor should be there, just do not use too much (between 1% and 2% of 500-600L is fine, 1% is mainly for color and more than 2% can make the roasted flavor and color excessive depending on taste, though). In Ireland and surroundings they barely know such thing as 300L roast, everything they do is ~500+L
2) do not use excessive amount of crystal, especially dark crystal as it makes your beer deep brown masking the red
3) avoid haze that immediately makes it opaque, you could lager or use kettle / fermenter finings and improve the process
4) do not make it too strong (people do this all the time and complain about the result), it is a light ale style and too dense wort is deeper in color than a light wort
5) not many 'Irish reds' are strikingly red, many beers in this category are just reddish brown, some are just brown and some are rather dark depending on the exact grain bill. It could be a nice drink even if not so red. You just need a little bit of roasted character or it is going to be one more bitter without adequate hops..
Basically, you could do it with Maris Otter base + 600L roasted barley, but you can add a little bit (3-6%) of crystal to increase body or 5% wheat to increase head (something that is not typical traditionally but could be nice looking, though). Something like Simpsons red rye crystal worked pretty well here but you must be very careful with the amount cause the color is provided mainly by roasted barley and using too much deep crystal will make it darker, as already said.
