I also live in Northern Ireland and can concur. I also live on a busy transatlantic flight path with both west and eastbound flights throughout the 24 hour day. From day to day, the sound from overflying high altitude aircraft does vary from nothing on occasions to rather loud at times.
There are a large number of factors at play here. The main one is background noise which varies largely due to the time of day. This includes such things as road traffic, farm traffic, machines running, house equipment such as boilers, immersion heaters, air conditioning running, even your refrigerator, water running, rain falling, the affect of wind on trees and buildings, drafts, people talking, TV
/radio, farm and domestic or wild animal sounds etc etc.
Clearly, the early hours of the morning on a calm night will generally see the lowest background noise.
Sound propagation in air is also affected by weather conditions including air pressure, ground air temperature, air temperature and pressure curve from altitude to ground, moisture content etc.
Wind direction and speed is another big factor. Also the variation in speed and direction of wind at the various altitudes between the aircraft and your location on the ground have an affect.
A further atmospheric factor that is important is how smooth or turbulent the air is both at the level the aircraft is flying at as well as the various levels between the aircraft's altitude and the ground.
Other factors can adjust the intensity of the sound from overhead airliners. Cloud has an affect, ie. single layer, thick layer, many layers, cloud altitude, cloud type etc. Also surrounding buildings or trees which may reflect or even focus sound at certain precise points on the ground. Surrounding hills or mountains can also reflect and even focus high altitude sound intensifying the same sound at certain points on the ground. I have been aware of this affect many times.
Some of the more obvious factors are the aircraft type, type of engine, engine power setting, whether it is climbing, cruising or descending. Also airspeed of the aircraft as well as ground speed are important in how the sound reaches any single stationary point on the surface below.
Remember that much of the sound you hear from an overflying aircraft, especially one flying at cruise speed, does not come from the engines but the aerodynamic affect of the air frame cutting through the surrounding air, or put another way, the airflow over the aircraft. This will also vary dramatically between flights even with the same precise aircraft type depending on how it is loaded, ie light, or at close to maximum payload.
It is not at all shocking that the UPS Boeing 767-300F which flew over you sometime after the Transaero Boeing 747-400 had dramatically different sound. Some time, albeit short, had elapsed hence many of the factors above could have changed if only momentarily. The different aircraft type and likely different engine types and ratings would have had only a small effect. I would suggest that the different altitude (5000 ft) was a big factor. Also the airspeed and ground speed as well as direction of flight in relation to you were very significant.
At the point over the ground where the two plane's paths crossed, which I understand is the vicinity where you were at the time, TSO554 was flying at FL370 at 563 knots whereas UPS232 was flying at FL320 at 393 knots. I haven't checked with the website to see what speeds are being quoted here but the large difference between the two aircraft speeds clearly suggests that these are ground speeds. Herein lies one of the main factors as to why the two planes were heard dramatically differently on the ground. It appears the Transaero was flying with a tail wind (generally the case with transatlantic eastbound flights). This would certainly be the main reason why you likely heard the sound before the plane reached you. Contrarily, the UPS was fighting a strong headwind making it less likely you would hear it in advance.
Finally, did either airplane fly precisely overhead, or did one or other fly ever so slightly north, south, east or west of your point on the ground? Combined with high altitude wind, this could make a big difference.
Even though my list of factors is long, there are still more but I don't wish this post to become too boring or no-one will read it!
As you can see so many things are at play. At the moment the Transaero aircraft flew over you, most of the above factors must have culminated at that precise moment to maximize the sound you heard.
Sorry for the post becoming unintentionally complicated, but now you have your answer. Hope this helps explain the seemingly strange anomaly.