Yip, estuaries in Ireland would be associated mainly with sea tout and bass (or mullet, but fly fishing for mullet will be that bit more specialised and might be best left for a later date..) but the predatory species like pollack, coley, bass, sea trout, wrasse and mackerel for example can most certainly be targeted from a range of coastline types inc rocks, piers and beaches.
It all comes down to finding suitable and comfortable areas to fly fish that will hold these fish types. Ideal ('suitable') areas are environments that offer appropriate depth of water (not too deep) and features like rocks, out-crops, drop-offs or weed beds etc where you might know where these fish can be found. You want to be able to get a back cast and if wading then you'd want to wade at a comfortable depth and over safe ground; ideally, you'd want shelter from wind too..
You'd be best with a fast intermediate or a sinking line, use pretty heavy tippet like 10-20lb. Simple clouser and streamer patterns of any and all designs will get you started.
It really is pretty straight forward after that. There is a wealth of information online, for example:https://www.peaksflyfishing.com/saltwat ... ers-guide/https://www.ukbass.com/fly-fishing-for- ... the-shore/
And for starters you could watch all the videos made by this guy:https://www.youtube.com/user/surethingsfly/videos