Can you use both a U.S. and Mexican passport while traveling?

I'm a dual citizen traveling to Ireland. Which passport should I use?

• I'm an American citizen and a British citizen. I'm traveling to Ireland soon and figured I would use my American passport in America and my British one in Ireland so I can skip immigration and all that. Are there any downsides to doing this?

• Answer:

  That's perfectly right, most dual citizens do that and I can only see advantages. European Community legislation provides that every Union citizen has the right to enter any EU Member State with a valid passport. It is irrelevant whether you are travelling for professional or private reasons, whether you are going to work in an employed or self-employed capacity or whether you are simply a tourist. Beside the requirement to hold a valid identity card or passport, there are no other conditions or formalities for the purpose of entry to the host Member State. However, the immigration officers are entitled to check the validity of your identity card or passport. Immigration officers may not: • require you to present an identity card with machine readable zone, a recent model of identity card or a passport with validity at least another 3 months … if the identity card or passport is valid, that is all you need; • ask you to produce any documents other than a valid identity card or passport, such as to furnish proof of means to support yourself, travel tickets, employment certificate, pay slips, bank statements, proof of accommodation, means of subsistence, medical certificate … • ask you how much money you have to spend; • ask you questions regarding the purpose and duration of your trip; • affix an entry stamp on your identity document or passport on arrival authorizing you to enter the national territory. http://ec.europa.eu/youreurope/nav/en/citizens/travelling/entry-procedures/for-union-citizens/index_en.html ..