It will still be hot end of Aug and into Sept
Make sure your accommodations have AC
I can’t imagine doing only 1 night in Venice- that’s hardly time enough to see the major sights and no time at all to experience the real Venice-quiet back canals and campos, the islands, etc
It’s recommended to fly INTo Venice because it’s a great place to get over jet lag and a pain to depart from that airport, arrival much easier.
Look for multi city or open jaw flights flying into one city out if another so you don’t waste time and money backtracking
Amalfi will also be crowded and hot it is lovely but requires a bit more effort to get there and transit around. Needs a good 4-6 days if you want to see Pompeii, Capri, coastal towns.
Fly into Rome out of Naples if possible
CT will be extremely crowded
Another option would be Paris then south to Provence/Nice
Fly into Paris out of Nice
We’ve been to Ireland twice while we did enjoy didn’t find it near as interesting as other European destinations- and we are both Irish descent. As for romantic hard to say, although I did come home pregnant from Ireland 32 yrs ago lol (Guinness is good for you)