Multi City Visit

Internal flights in US are generally expensive, forget about Ryanair prices. With covid it is hard to plan for next year. If it was me - I would make up my mind about cities I want to visit first. Definitely wouldn’t bother flying between Boston and NY, it’s only 4 hours by car/bus, you can rent a car one way fairly cheap, that route is very straight forward or bus fare can be got for half nothing. Chicago is a bit further, decide which city you ll be flying from and start monitoring, use skyscanner and check out nearby airports, like for Boston it can be Providence. Frontier, Southwest Airlines, Spirit, Jet Blue are some of airlines to check. If you don’t mind flying from London - you were able to get amazing deals before, no one knows how it’s going to be next year We flew last August Boston Logan-London Heathrow with Virgin Atlantic for $325 return, hand luggage only, but food and drinks were included. Check out app called Hopper, it tells you if it’s a good price for your flight and if you should buy or wait. I found it very useful.
Now, in saying all that, start with doing multi city search on Aerlingus site, they are partners with Jet Blue and sometimes their prices are great. My in-laws flew Shannon-Tampa via Boston on the way over and NY on the way back in March and their tickets were cheaper than for us to fly from Boston to Tampa ��

Try the multi city function on Google flights and buy direct from the airline. SouthWest and JetBlue have some good fares but they don't always use main airports.