Antrim and Westport for 10 days?

Five days seems like a long time to stay in Antrim. And Westport is a long way from there.
Also, Antrim seems a long way to drive coming off a transatlantic flight. Maybe head to Belfast for a day or two and pick up the car there?
The highlights of Northern Ireland and the northern parts of Ireland, IMO, are:
Belfast
Giant's Causeway and Dun Luce castle ruin
Derry/Londonderry
County Donegal, including Slieve League, Killy Begs is a good place to stay to see the cliffs from both land and sea
I haven't been, but a lot of people like the Ulster American Folk Park near Omagh
On a trip of this length, I'm not sure I'd go further south than Sligo on the west coast. Nearby is the Carrowmore Megalithic Cemetery, which is on my list.

All good advice from G3rryCee. I would only add a few thoughts:
Make sure that your rental car company knows that you'll be taking the vehicle across the international border. Some providers have started adding a nominal fee for that, and you'll want to ensure that it's included in your contract to ensure there's no inadvertant lapse in your insurance cover.
Know too that NI, and the UK in general, now requires an ETA for travel in the country. You'll want to ensure that you have one in the unlikely event that you have any interaction with local law enforcement.
Lastly, the All Trails App works great for Ireland. We found the descriptions of the various hikes, their degree of difficulty, and user reviews all to be quite helpful in planning our own hiking adventures in NI, Donegal, Sligo and Connemara.
I can recommend the Ulster-American Folk Park - it's extremely well done and is particularly educational if your ancestry traces to Ulster - maybe study their website before making up your mind about whether the side trip would be worthwhile.

Depending on how bad you want to hear music (and have a beer while listening), you need to base in spots with regular music, where you can walk to your lodging (or have a designated driver). Were headed to Antrim (Cushendall) next Spring for two nights, and I don't think you'll find much music with Belfast right next door. Cushendall has music some weekends, but nearby towns don't mention such a thing. Five nights is a big investment unless you plan to visit all the glens.

Westport is a reasonable choice, music wise. You can consume five days if you don't mind ranging over a large area. Matt Molloy's and Porter House have music 7 nights a week, although it might start drying up in September.

We're in the "travel light, move regularly" camp. In five trips we've never spent more than three nights in one place, and very few of those. Most of the stuff we enjoy is between towns, and I'll happily relocate if there's music down the road. We don't "settle into" lodging, since we're always on the go and usually leave the lodging after breakfast and return when the bars close. I'll be 77 next year, so maybe I'll see the light of long stay s:-)

As an example, our next N. Ireland trip will be Cavan, Omagh, Belfast(2), Cushendall(2), Derry, Letterkenny, Tory Island, Sligo(2), Belmullet(2), Clare Island, Inishbofin(2), Cong, Galway, and Trim. Belfast and Galway are re-visits.

Thanks for this helpful advice. We are still formulating plans.

We’ve been to Dingle and Galway, but maybe we should expand our sights beyond the north. We are pathologically flexible and impulsive travelers, so we are used to turning on a dime!

And one more question: after we land in Dublin, before we head out anywhere, we plan to stop at Newgrange. Will 2 days be sufficient?

Will two days be sufficient for what? The Newgrange tour takes probably half a day, but there is more to see in the area. It makes sense to head north from the airport to somewhere close by for two nights while you adjust to the time change. A few years ago, we stayed here:
https://www.airbnb.com/rooms/10185982?source_impression_id=p3_1754577758_P3IkSy8wqA3vh7ik
the photos are not great. The second bedroom is in a mostly open loft that is accessed by sturdy but narrow stairs. We really enjoyed it. The location was great, near Monasterboice.
Be sure to book Newgrange in advance. I would not book it for first thing the next morning unless you're sure everyone will be up and at 'em.

Using Westport as a base is an excellent idea for hiking, history and food. Also, using Antrim for is an excellent. However, I only passed through Antrim once over a couple hours. I spent several days in Westport and nearby County Sligo and thus know it well.

You mention hiking: I strongly recommend a climb of Croagh Patrick (https://www.mayo.ie/attractions/croagh-patrick), the so-called holy mountain of Ireland. It's only about 2,500 ft. high with awesome views of Clew Bay and the Atlantic. I climbed it in 2.5 hours in 1997. It's not paved, but the footing is generally good. Access to the mountain is only about 5 miles west of Westport in Louisburgh.

You could also take day trip hikes along the Connemara Way, south of Westport: https://www.go-to-ireland.com/what-to-see/connemara-way/. I have walked the Dingle Way in southern Ireland and the Wicklow Way in eastern Ireland: The way-marked trails of Ireland are well maintained and picturesque.

Another hiking idea is to visit Glenveagh National Park in County Donegal in the Republic: https://www.nationalparks.ie/glenveagh/. You could access this from either Antrim or Wesport, but Westport is probably better.

Thank you all for your helpful and inspiring responses!

We spent a full day in the Boyne Valley, getting the first tour of the morning of Newgrange/Knowth, then visited Monasterboice, Tara, and The Boyne Museum (which was excellent). We did it as a day trip (with car) from Dublin.
I don't know how busy Newgrange is in September, but make your reservations ASAP, as the tour is the only access.

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yup, you will be entering the United Kingdom across the land border, just because there are no fixed checks doesnt change the fact that you will need a ETA to enter the UK (Northern Ireland) across the land frontier , also the UK drives in miles per hour , Ireland drives in Kilometres per hour, also be aware of the change of this at the border, also below i have posted the UK highway code for Northern Ireland https://www.nidirect.gov.uk/articles/highway-code
remember the currency in Northern Ireland like the rest of the UK is pounds and pence , not euros !

also there is nothing in antrim it is abit of a dump TBH, are you talking about the county or the county town of the same name
also regarding names Just call Northern Ireland , Northern Ireland , not the Northern part of Ireland, people will soon correct you and tell you Northern Ireland and Ireland are two different countries (which they are of course NI being part of the UK)