Home Insulation - Where to start?

Start with another 250mm of mineral wool on top of what's there. Ensure you don't block the ventilation path at the eaves. Thermal imaging just gives you an indication of possible heat loss areas but this still has to be confirmed by someone who is experienced in house surveys. (Not insulation installers)

Is there a chance it was occupied before 2006?

If it was you can contact the SEAI for advice about upgrades and available grants, should be some free advice anyway

If it was me id do:

The attic insulation suggested above and some insulation & sealing around your attic hatch but be aware the more you insulate the attic the less heat gets to your water tank so you might need to insulated this also

Then closely check all the seals & closing locks for windows and doors and get them replaced/realigned if necessary

Then you are onto the big boy stuff such as pumping cavity, external insulation and/or insulated slabs to the inside leaf of all external walls which are all decent costs if you cant avail of grants or know a builder!

Have a 1998 semi detached with a B3 rating. I topped up the attic insulation by 200mm and changed the windows on the side of the house to triple glazed since the last BER. Front and rear windows were already changed to triple glazing in 2017 when we built an open plan kitchen extension out back. The hall door is a Palladio double glazed unit, we have no porch. The hallway has always been much colder than the rest of the house and I’d always put that down to the original double glazed windows. This week I decided to investigate further after noticing that the internal hall wall below the stairs was ice cold to the touch. The remote control for the gas fire has a built in thermometer so I left it in the hall for the evening and checked the readings which were 15 deg at floor level and 16 deg at the top of the newel post. The rest of the house was reading at 20-21 deg depending on location. I took off the skirting board between the newel post and toilet door frame and cut out a section of the plasterboard to discover that the outside wall below the stairs was just bare block and that cold air was entering through the gap between the stair and wall. I removed a larger section of plasterboard and a vertical stud in order to gain access then installed two layers of 50mm insulation on the outside wall making sure that the first layer fitted up into the gap between the stairs and the outside wall. I installed another 100mm of insulation board on the back wall of the below stairs toilet and fixed everything into place with expanding foam. The original plasterboard used in the house had a polystyrene foam backing and these sheets were bonded to the walls leaving an air gap of around 10-12mm. I can only assume that cold air was being drawn down from the attic through this air gap and escaping into the void below the stairs. The temperature in the hallway is now a constant 19-20 degrees and there is no longer a draught when the door between the kitchen and hallway is left open.