5 months in europe on a serious budget - should i buy a eurail pass?

If you activate the pass the day before, in London, you can use it to get a reduced fare on the EuroStar.

It might make more sense to get a flight directly from the U.S. to Portugal, then come back via Paris, London, Ireland.

PS: you might want to delete the duplicate posting before people start replying to both.

If you have not already done so you need to be sure that your proposed itinerary is compatible with the Schengen visa requirements.

You could fly from Kerry to London Luton on Ryanair, spend the night in the airport, and take a morning easyJet flight to Lisbon. Or you could go to Dublin, spend the night, and take a morning Aer Lingus flight to Lisbon.

However, Jeff's point about the Schengen rule is so important that I'll add more details. If you don't know it, you can stay in the Schengen zone a total of 90 days in any 180-day period. Spending five months in Europe means that you'll have to spend two of those months outside of Schengen zone countries. While the United Kingdom, Ireland, and Croatia are not Schengen countries, the others on your list are.

Elizabeth,
The note on Schengen requirements is critical to you. You may NOT stay more than 3 out of any sixth months without incurring terrific fines. We paid over $500 for our daughter overstaying by 3 days. They will catch you at an airport, even on leaving. They can also take you aside for a long talk, forcing you to miss your flight home and having to pay full freight for a new flight (one that leaves immediately). The Schengen requirements are serious business. If you do not pay the fine they have the right to exclude you from Schengen travel for up to ten years. You need to research this, and ignore the travelers who advise you to take your chances.

Agree w/ the previous replies as to Schengen requirements.

If you want to be in Europe, on a budget, and need to spend 2 full months out of the zone, you'll want to spend the balance of that in Croatia and the Balkans. Definatly some worthy island hopping in Croatia, and some other neat destinations in Bosnia/Serbia/Montenegro worth checking out - and they're all budget friendly.

The UK, on the other hand, isn't.

Another easy out of Schengen zone escape from Portugal - Morocco!

Thank you so much to everyone who has called my attention to the Schengen zone. I can't believe I had no idea about these rules so close to my departure. I am going to try to stretch out my stays in Ireland and croatia and will likely have to change my return flight. Again, thank you so much for all the informative responses, what a great forum.
Elizabeth

"i will be doing work-trade in ireland and italy"

Presume you have the appropriate visa to allow you to do the above? A tourist visa will not be enough.