sole trader trade or professional income is assessed under Schedule D of the tax code not Schedule E as for PAYE income. Therefore a Form 11 is appropriate. If you had self-employed income in 2006 you need to act on filing this ASAP. Paper deadline is 31/10/07 and ROS filing/payment is 15/11/07. Get your accountant onto it immediately. Surcharges apply for late submissions.
SOmetimes income of a minor nature can be dealt with on a Form 12 but while you say it was less than €15,000, I assume it's more than a few hundred. The maximum I've found Revenue accepting on a Form 12 is about €1,500-€2,000 and then where it's largely once off. If your trade is continuous then you need to be filing on Form 11.
Thanks for the response, I'll contact the accountant immediately. Though from reading this thread, is the deadline of 31/10/07 for 2006 return not applicable, i.e. I have in theory until 2008? though in practice wish to keep on top of the returns rather than leave it until later...