I think the most popular first stop from Milford is Kilmore Quay. When I last went across this was our first stop before we then sailed up the coast to Arklow and then back across to Fishguard.
You don't say what sort of keel you have and how much time you have!.
The east coast has only a few harbours. You can moor against the wall at Rosslare but its a bleak place. Then Arklow and Wicklow going north and then your at Dublin. The sw coast of Ireland is nicer. Try Kilmore Quay (Kehoes pub closed but another 5mins beyond at top of hill), then Waterford, Gungarvon (dries) Ballycotton, Crosshaven or East Ferry in Cork, Oysterhaven, Kinsale. After that there is dozens of harbours to visit.