It all depends on your ground and weather, as its been grazed over winter I'll assume its dry now so go ahead and harrow. This will break up the thatch and tear out moss.
Next job is take a handfull of topsoil (inn a bag) to your local agricultural collage to analyse.
You will want Lime and I would suggest over liming, the college will tell you how much you need to apply pwer acre to rebalance the Ph of the soil but if you up this to 3t per acre you will have the benefit of killing off bacteria including coccidiosis (particularly if you have both poultry and equines).
This can be done as soon as you have harrowed, as long as you are not expecting significant rainfall which will wash the lime off the land quyicker than it can be absorbed through the surface(this is where harrowing is helpfull as it damages this initial crust allowing quicker absorbtion) follow the application with a light roll if you have the option.
Once Sol is making an effort and the ground warmed above 7degC the grass will start to grow and you can start thinking about fertiliser, re-harrowing the day before and rolling agin after is parramount if you want to see the most for your money.
How old and what type is your lay? I have the exact same situation here and when I spread the Fert I will be adding grass seed to it for an over-seed to build up the sward as our lay is now 5years on and was not as strong to establish as I would have liked.