By Air
Boston Logan International Airport is the closest airport to Cohasset and would be your best option if you are travelling by air.
Travelling from Ireland
If you are travelling to Boston from Ireland, there are direct flights with Aer Lingus from both Dublin and Shannon airports. Flights out of Shannon tend to be cheaper than out of Dublin so make sure to check both options.
There are other airlines that fly out of Dublin too that will get you to Boston. They will involve a connection in another airport.
Those airlines are:
The following booking engines will allow you to search all flight options at the same time which will assist you in finding the best deal but remember that they will not all be direct flights:
Travelling internally in the US
Boston Logan Airport is a hub for all the major airlines so there are direct flight options from every part of the country. Listed above are the main airlines but dont forget JetBlue, Southwest and Spirit as they do not always show up in Kayak or Orbitz.