An Irish trademark is registered with the Intellectual Property Office of Ireland (IPOI). It is valid only in Ireland. In comparison, an EU trademark is registered with the European Union Intellectual Property Office (EUIPO) and is valid across Ireland and 26 remaining member states of the European Union. So both would be valid in Ireland, but the EU one is also valid across the rest of the EU.
Besides validity, there's also a difference in price and the length required to assess your trademark.
The initial application fee for an Irish trademark is €70 plus €70 for each additional class of goods and services. The base fee for an EU trademark is €850 (online application), with €50 for the second class and €150 for the third and each additional class. Regarding time, it takes about 6 months to get an Irish trademark and about 4 to get an EU one.
To summarise, if you don't have clients outside of Ireland, you don't need an EU trademark; however, you might want to secure it if you plan to expand across borders in the future. If you wish to stay local, an Irish trademark offers one more advantage besides the lower price - your application will likely have less chance of being opposed by other trademark owners.