Clearly there were many factors which helped to precipitate the current crisis, including numerous domestic political and policy mistakes. However, being a member of the single currency definitely seems to have played a part.
According to an interesting paper published by European Commission officials, joining the euro had the following consequences:
"The Portuguese economy went from a boom led by in the second half of the nineties to a marked slowdown in this decade (Chart 1). A major impulse for the expansion was the considerable fall in interest rates when the prospect of accession to EMU became increasingly self-validating. Nominal short-term interest rates fully converged to those at the common low level set by the ECB (Chart 2)."The paper suggests that the "substantial fall in interest rates, [was] the main trigger for the boom", which later resulted in a bust. The interesting difference with Portugal, compared to Spain, Ireland and Greece, is that it experienced the infamous boom - bust dynamics before it entered the eurozone, but still because of the "prospect of accession to EMU", as the officials put it.
The eurozone's "sleeping pill" dynamics (as Herman Van Rompuy puts it) have prevented investors from forcing the country into unpopular, but necessary, reforms to boost competitiveness, resulting in Portugal becoming 21% less competitive relative to Germany over the past decade (this looks to be true of the past and current crisis).
So when Portugal experienced its bust, long before Spain and Ireland experienced theirs, there wasn't a currency which could come under strain and thereby discipline the politicians. This was surely one of the reasons for the consistently sluggish growth in Portugal following its bust.
There is, obviously, more to it though. Thanks to the ECB's artificially low interest rates, which were designed for a slow growing Germany, the country started accumulating very high levels of private debt, up to more than 200 percent of GDP (just like in Spain and Ireland, however, they managed to grow at the same time ).
On top of all of this, the Portuguese government hasn't exactly been a shining example of sound budgetary management. It has run large budget deficits for many years and has accumulated a significant level of debt for an economy the size of Portugal's (both the debt and deficit figures for last year were recently revised upwards as well).
Fundamentally however, an overvalued currency which prevented growth (the Escudo would have decreased in value following the original bust), combined with interest rates which were an ongoing stimulus to take on new debt, left Portugal with a decade of low growth. The resulting fall in relative living standards, compared to the rest of the EU, and the fall in tax receipts further fuelled the build up of both private and pubic debt.
Interestingly, Portugal's problems can provide some hint at what the future may hold for Greece, Ireland and possibly Spain. Even loose monetary policy and boosts in liquidity (admittedly in debt form) didn't solve Portugal's underlying competitiveness and currency problems, in fact they may have made things worse.
These lessons from Portugal's past should be heeded by those in charge. Tackling the root causes of this crisis - eurozone imbalances, competitiveness problems and the banking crisis - is of paramount importance, as is providing for all eventualities, including a restructuring and possibly even a change in eurozone membership.