You'll be fine
.
As long as you didn't overstay your previous visitor visa (which is 3 months if you enter through Ireland), and you are honest about your trip, you shouldn't have any issues.
Really, the only times it can look suspicious if entering through Ireland as a visitor are:
- if you have been refused a UK visa in the past
- if you have been refused entry to the UK in the past
- if you have spent several months in the UK recently, or have visited multiple times in a short period (i.e. 6 months or more in the last few months) and have been warned about spending too much time here
- you overstayed your previous visitor visa through Ireland (i.e tried to stay 6 months in the UK when an Irish visitor visa only allows 3 months)
Because those could be reasons why you might try to deliberately enter through Ireland to avoid UK immigration.
Also, if you have been issued a visa valid for residency in the UK (i.e. a student, work or spousal visa), you also don't want to enter through Ireland because your visa won't be stamped to show when you arrived... and it's important to get the stamp.
In your case, you just went for the cheapest option, which happened to be through Ireland, and you were unaware of the lack of border controls between Ireland and the UK. You left before the visa expires, so you did nothing wrong at all.
Just to be safe, when you arrive in January, I would have with you:
- your return ticket
- a bank statement showing you have enough money for your trip
- evidence of your job in the US
- evidence of your home in the US
and also having knowledge of the immigration rules (i.e. that you have to return to the US to apply for a spousal visa) is helpful too.