Shipping belongings to and from Europe

The short answer is that there won't be any that I consider affordable, but you may disagree. I think you'll be able to get estimated shipping costs from the websites of USPS, DHL, and UPS.

You mention spending 4 months in Europe. Are you aware that you are limited to no more than 90 days (counting arrival day and departure day) in the Schengen Zone? Most of western Europe falls in that area, but Ireland and the United Kingdom do not, so you may be OK. Do not take any chances. The penalty for overstaying the limit can be a sizable fine and banning from the Zone for a number of years.

Echoing acraven's reminder about the 90 stay limits, I have used Mailboxes Etc with success on several occasions. They aren't cheap, but they are reliable and sort of reasonable.

I've used Send my Bag to ship a bag from UK to US. Very efficient and it arrived in two days. Not cheap but reliable.

It used to be as cheap to send stuff from an overseas military base as in the states. It's been awhile since I was stationed overseas and didn't do a lot of shipping even then, it might be worth looking into. I think you can sign into a base as a visitor and I don't recall the Post Office (USPS) requiring proof you are military. The hard thing now is finding a US base, they're a lot fewer and further between.

Thanks for the info. We didn't know about the Schengen Visa. From what the embassies told us we should be OK if we leave Spain within 90 day window. Phew. I'll check into your shipping sources. Would just buy gear overseas but we want to do some hiking in Ireland. Hate to break in new boots, pack a, etc.

With Schengen it is actually very simple.

As far as boundary crossing, within the Schengen area, there are normally no checkpoints or checks. People can move freely over the border as much as they like, like going from Texas to California (without the Arizona Ag inspection). You drive along or ride on the train and all of a sudden the language changes, and if you are in a car there is a wide place in the road where the border checks used to be, there will be a welcome to Country sign showing the national speed limits in that country and the road surface will change.

You get checked once as you enter the Schengen area and once when you leave for every time you go back and forth into it. Most of Europe is in the Schengen area, but Ireland and the UK are among those who are not.

If you are travelling from North America you are entitled to be in the Schengen Zone up to 90 days in any 180 days, including any part of any arrival day and any part of a departure day.

Ireland will have its own checks and normally grants up to 6 months.

Where else will you visit in your 4 months besides Ireland and Spain? Are you a US or Canadian Citizen travelling on a US or Canadian passport?

You haven't completed anything on your profile here and neither of your posts give any indication which continent or country you are travelling from. The rules are different for various countries of origin.

Don't forget that both your arrival day and your departure day count when you are figuring your Schengen time, and leaving the zone does not reset the timer to zero; it just pauses the count.

From what the embassies told us we should be OK if we leave Spain within 90 day window.

Just to reiterate what the others say, the limit is 90 days (in 180) in the Schengen Area as a whole (unless you are an EU or Schengen Area citizen). You can't spend 90 days in Spain and then a few in France, you have to go totally outside the Schengen Area and not come back until you have less than 90 days in the last 180.
See this map here: https://ec.europa.eu/home-affairs/sites/homeaffairs/files/schengen_map.jpg

And arrival and departures dates count as full days. Departing on day 90 is risky.

Day 90 is really risky. Aside from illness, your planned transportation might be affected by weather or a strike. I now shoot for Day 88.

Thanks for all the wonderful advice. This blog is a wonderful resource. We're planning to head to Ireland a couple weeks before the 90 days is up. Just to be on the safe side. Do you know if the 180 day clock starts from when you land or when you leave?

The 90 day clock starts the instant you enter territory within the Schengen area. If it is one minute before midnight, that day counts and then the next morning will be day number two.

Arrival and departure days count.

The 180 is very very easy. Count back from today 180 days. In those 180 days have you been in the Schengen area less than 90 days? Yes? Then you can stay another. No? Then you are illegal.

There is no start on the 180 days. It rolls from day to day. You always count back 180 and see if actual counting days is less than 90.

When planning, you don't need to worry about 180 until you have reached 90. It won't be any problem until you get to 90. If you are planning a trip of 120 days you just know that some time before you reach 90 you need to have some combination of at least 30 days outside the area - not forgetting arrival and departure days.

The 180 day clock starts at your first point of entry into the Schengen zone. Arriving from the US, that would be the day you land and your passport is entered into system. Passports aren't always stamped anymore, but they are scanned, so you can't always find your entry date by looking at your passport; keep track of it yourself.

Do you know if the 180 day clock starts from when you land or when you leave?

Neither. The limit is exactly as it says "90 days in 180",. Some people call it a "rolling limit".
You must not, at any time, have more than 90 days in the Schengen Area in the last 180. So, every morning, you ask yourself "how many days, in the last 180, have I been in the Schengen Area?". If the answer is 90, you must leave that day. And you cannot re-enter the Schengen Area until some days are more than 180 days in the past so your total "in the last 180 days" goes under 90.

And of course arrival and departure days count as full days.

The official rule is here, from the EU's own website (see section 2): https://eeas.europa.eu/sites/eeas/files/visa_waiver_faqs_en.pdf
Trust no other website, many have it wrong.

We are U.S. citizens and will be traveling from the States. The EU link didn't work for me. I'll try thy he website directly. Thanks again for all the guidance.