Plans for a single currency were first agreed in the Maastricht Treaty
of 1992. The euro began life on January 1, 1999, when exchange rates
with "legacy" currencies were irrevocably fixed. For the first two years
of its existence, it was only an electronic currency used by banks. The
creation of Euro was established in the Maastricht Treaty (1992).
To participate in the currency, the member states had to meet some
strict criteria as a debt ratio of less than 60% of their gross domestic
product (GDP), interest rates close to EU average and a budget deficit
of less than 3%.
The first notes and coins were issued on January 1, 2002, when the
euro became legal tender for all transactions in all 12 countries. The
old national currencies were phased out over the next few months.
1) Joining the euro introduced all euro zone countries to
the European Union's (EU) internal market (sometimes known
as the single market) and seeks to guarantee the free
movement of goods, capital, services, and people – the
EU's "four freedoms" – within the EU's 28 member states.
This is good for Ireland as we can Trade our products with all
other Euro zone countries whose population is estimated to be
just over 500 million people. This is also good as well also do not
have to relay mainly on the UK which we did in the past. 
Prices are made easier for people to deal with
when they are in a foreign country as all the
prices are the same as they are at home.
It means less confusion for personal
purchases and business deals.
The Euro also made Europe a powerful player
in the economy and the 2nd most important
currency in the world after the Dollar. 
Having many countries with the same
currency, companies have more competition
with other companies in different countries
which means they may make prices cheaper
for their product than their comparators
which is good for the consumer you get the
best price for your product 
10 years leading up to the Euro all countries had to have:
Low Interest and Inflation Rates
Stable currency for 2 Years
Good Govt Spending and low Govt Borrowing
There was concern about the PIGS (Portugal, Italy, Greece
and Spain ) but economies with this criteria were allowed
join and this was viewed as healthy economies working
together could be nothing but good. 
Some PIGS countries would devalue their
currency so they would be viewed as cheap
countries to do business with and this was
not fair for competition.
Joining the Euro zone meant this could not
happen and business would be a fair trading
Most big companies want a European base
and being part of the euro was appealing to
them as we all had a common currency.
This helped Ireland attract FDI such as
Facebook and Twitter. 
Joining the Eurozone meant that all countries
lost the ability to control their own interest
Last Thursday (7th November 2013) The ECB
(European Central Bank) cut the interest rates
to a new record low.
The rate was cut to 0.25% from 0.5%. This
will means a saving of about €30 a month for
a mortgage holder with a €250,000 home
The move was a response to a slump in
inflation way below the ECB target that has
sparked fears the euro zone's economic
recovery could stall.
Not having control of our country's own
interest rates can make it difficult for an
economy to recover from financial
depression. Not all countries suit the same
rate at the same time. E.g. Greece and
The Single Market and Tomorrows Europe, Mario Monti, Kogan Page,
1996, pg, 77- 95
Information retrieved on (7/11/13)