Aon use Irish Life as their fund manager, so both pensions are most probably invested with the same life company. The difference in fund performance is the funds. You haven't said where either of them are invested? And remember, there is a relationship between risk and return so maybe the Irish Life one is invested in riskier assets, hence the better return.
Whether to transfer to your Aon scheme, look at the management fees under both first.
Steven
http://www.bluewaterfp.ie (www.bluewaterfp.ie)