You may be entitled to more than you think. This is slightly complicated by the fact that you clearly booked the flight through United. However, as the flight was operated by Aer Lingus, which is an EU carrier, you may be entitled to €600 compensation for each cancelled flight, under regulation 261/2004. This applies to flights which originate in Europe or are operated by a EU carrier. The fact that the delay was caused by weather, mechanical problems or volcano's is irrelevant. Write to United and assert your rights under this regulation, stating specifically that the regulation applies becuase the flight was operated by Aer Lingus and this is an EU based carrier. You are entitled to this compensation as cash and you do not have to accept vouchers. If you have problems, you can link to the IAA website which regulates Aer Lingus.
For details of regulation 261/2004 and how it is implemented in Ireland, which is the regulating authority for Aer Lingus follow this link
http://www.aviationreg.ie/FAQs/Default.213.html
If United reject your claim, you can file a complaint online with the Aviation Regulator in Ireland, which is the compentant authority for the enforcement of 261/2004 regulations. This can be found at
http://www.aviationreg.ie/
They will make a ruling in your favour. You can then enforce this through Small Claims Court. You can also file this online, at the following address:
https://smallclaims.courts.ie/esmall...nguage=English
To summarize take the following steps:
1. Write to United and assert your claim under EU Reg 261/2004. State specifically that you believe this regulation applies because the flight was operated by Aer Lingus and you will pursue your remedy with the regulatory authorities and/or small claims if they do not pay your compensation as provided under this regulation.
2. Wait until United reject your claim, then file with the Aviation Regulator at the above address, enclosing all correspondence and details. if the regulator finds in your favour, send their finding to United and demand immediate payment.
3. If payment is not forthcoming, file an electronic claim in the Small Claims Court in Ireland at the above link. Typically, the airline will back down and pay. If they don't you will have a default judgement against them and you can take pleasure in arranging for bailiffs to seize a jet at Dublin Airport until the bill and your costs are satisfied. Let me know if that happens, and I will fly there myself and video it.