Originally Posted by j2simpso

* Lounge access in the USA is very, very complicated and not for the faint of heart. To attempt to summarize the complex rules they are:

• Are you United Gold or above member?
  • Yes : Does your itinerary entirely consist of domestic segments with UA?
    • Yes : Check to see if there's a *Gold lounge not operated by UA per this spreadsheet (i.e. *A lounge @ LAX)
    • No (UA and Other * partners) : Access to UC granted
  • No : Are you a member of a *Gold member of another FFP (i.e. Aeroplan):
    • Yes : Access to United Clubs and all *Gold lounges granted
    • No: Are you travelling in BF on a Premium Service route (i.e. EWR -> SFO)?
      • Yes : Access to UC granted
      • No : Is there an international segment in J with a UA or a *A partner?
        • Yes : Does the segment cross continents (i.e. USA to Europe)?
          • Yes : Is that segment operated by UA?
            • Yes : Access to Polaris Lounge on arrival, departure and connecting airports
            • No : Access to Polaris Lounge on departure airport of your trans-continental journey
          • No : Access to United Club
        • No : No access provided

Again these rules are complex, even the most experienced UA flyer has been denied entry because they weren't aware of the nuances of these rules.

Generally speaking when discussing lounge access at the end of the day what matters is the operating carriers (not marketing carriers) of the ticket - in your case United and Aer Lingus. You would want to refer to the Aer Lingus lounge website which lists the following options:

• SFO : Any United Club in T3
• SEA : The Club at SEA (beside gate A11)

I'm unsure how SFO lounge access is granted since you are not leaving SFO on Air Lingus but rather out of SEA.

Hope that provides some guidance.

Safe Travels,

James